Update extent object based on the discussion in #168

Resolves #168

The text in Core does not have examples for the more complete cases that extensions may use. This would be how an example from the discussion in the issue would be represented:

```
{
  "extent":{
    "spatial": {
      "bbox": [[180, -56, -180, 83]],
      "crs":"http://www.opengis.net/def/crs/OGC/1.3/CRS84"
    },
    "temporal":{
      "interval": [["2015-06-23T00:00:00Z", "2015-06-24T00:00:00Z"], ["2015-06-23T00:00:00Z", "2015-06-24T00:00:00Z"]],
      "trs": "http://www.opengis.net/def/uom/ISO-8601/0/Gregorian"
    },
    "thermal":{
      "range": [0, 273.15],
      "uom": "http://www.opengis.net/def/uom/SI/kelvin"
    }
  }
}
```

core/openapi/schemas/extent.yaml

@@ -1,3 +1,90 @@
+description: >-
+  The extent of the features in the collection. In the Core only spatial and temporal
+  extents are specified. Extensions may add additional members to represent other
+  extents, for example, thermal or pressure ranges.
+type: object
+properties:
+  spatial:
+    description: >-
+      The spatial extent of the features in the collection.
+    type: object
+    properties:
+      bbox:
+        description: >-
+          One or more bounding boxes that describe the spatial extent of the dataset.
+          In the Core only a single bounding box is supported. Extensions may support
+          additional areas. If multiple areas are provided, the union of the bounding
+          boxes describes the spatial extent.
+        type: array
+        minItems: 1
+        items:
+          description: >-
+            West, south, east, north edges of the bounding box. The coordinates
+            are in the coordinate reference system specified in `crs`. By default
+            this is WGS84 longitude/latitude.
+          type: array
+          minItems: 4
+          maxItems: 6
+          items:
+            type: number
+          example:
+            - -180
+            - -90
+            - 180
+            - 90
+      crs:
+        description: >-
+          Coordinate reference system of the coordinates in the spatial extent
+          (property `bbox`). The default reference system is WGS84 longitude/latitude.
+          In the Core this is the only supported coordinate reference system.
+          Extensions may support additional coordinate reference systems and add
+          additional enum values.
+        type: string
+        enum:
+          - 'http://www.opengis.net/def/crs/OGC/1.3/CRS84'
+        default: 'http://www.opengis.net/def/crs/OGC/1.3/CRS84'
+  temporal:
+    description: >-
+      The temporal extent of the features in the collection.
+    type: object
+    properties:
+      bbox:
+        description: >-
+          One or more time intervals that describe the temporal extent of the dataset.
+          In the Core only a single time interval is supported. Extensions may support
+          multiple intervals. If multiple intervals are provided, the union of the
+          intervals describes the temporal extent.
+        type: array
+        minItems: 1
+        items:
+          description: >-
+            Begin and end times of the time interval. The timestamps
+            are in the coordinate reference system specified in `trs`. By default
+            this is the Gregorian calendar.
+          type: array
+          minItems: 2
+          maxItems: 2
+          items:
+            type: string
+            format: date-time
+          example:
+            - '2011-11-11T12:22:11Z'
+            - '2012-11-24T12:32:43Z'
+      trs:
+        description: >-
+          Coordinate reference system of the coordinates in the temporal extent
+          (property `interval`). The default reference system is the Gregorian calendar.
+          In the Core this is the only supported temporal reference system.
+          Extensions may support additional temporal reference systems and add
+          additional enum values.
+        type: string
+        enum:
+          - 'http://www.opengis.net/def/uom/ISO-8601/0/Gregorian'
+        default: 'http://www.opengis.net/def/uom/ISO-8601/0/Gregorian'
+
+TODO
+DELETE:
+
 type: object
 properties:
   crs:

core/standard/annex_ats.adoc

@@ -41,7 +41,7 @@ The WFS 3.0 standard does not mandate a standard format for the API Description
 
 ==== A.2.3 Resource Encodings
 
-The WFS 3.0 standard does not mandate a standard encoding for resources returned by the API. Yet a compliance test requires some minimal level of expected behavior. Therefore, this Abstract Test Suite asserts that the API returns resources encoded in HTML and GeoJSON. Since no compliance test suite exists for these encodings at this time, the resources shall be presented to the test operator for human inspection.  
+The WFS 3.0 standard does not mandate a standard encoding for resources returned by the API. Yet a compliance test requires some minimal level of expected behavior. Therefore, this Abstract Test Suite asserts that the API returns resources encoded in HTML and GeoJSON. Since no compliance test suite exists for these encodings at this time, the resources shall be presented to the test operator for human inspection.
 
 ==== A.2.4 Processing Security Objects
 
@@ -143,7 +143,7 @@ For each feature collection in this distribution of the dataset, the links prope
 All links SHALL include the rel and type properties. +
 Tests: A.4.4.6 +
 | *Requirement 14:* Feature Collections Metadata Extent +
-For each feature collection, the extent property, if provided, SHALL be a bounding box that includes all spatial and temporal geometries in this collection. +
+For each feature collection, the extent property, if provided, SHALL provide bounding boxes that include all spatial geometries and time intervals that include all temporal geometries in this collection. +
 If a feature has multiple properties with spatial or temporal information, it is the decision of the server whether only a single spatial or temporal geometry property is used to determine the extent or all relevant geometries. +
 Tests: A.4.4.6 +
 | *Requirement 15:* Feature Collection Metadta Operation +
@@ -250,7 +250,7 @@ none
 
 ====== c) Test Method:
 
-. All compliance tests shall be configured to use the HTTP 1.1 protocol exclusively.  
+. All compliance tests shall be configured to use the HTTP 1.1 protocol exclusively.
 
 ====== d) References:
 Requirement 7
@@ -462,17 +462,17 @@ None
 ===== A.4.4.1 Validate /api path
 
 ====== a) Test Purpose:
-Validate API definition provided through the /api path it the athoritative definition of this API.  Validate that this resource exists at the expected location and that it complies with the appropirate schema. 
+Validate API definition provided through the /api path it the athoritative definition of this API.  Validate that this resource exists at the expected location and that it complies with the appropirate schema.
 
 ====== b) Pre-conditions:
 * A URL to the server hosting the API definition document is known
 
 ====== c) Test Method:
 
 . Issue an HTTP GET request to the URL {server}/api
- 
+
 . Validate that a document was returned with a status code of 200
- 
+
 . Validate the returned document against the OpenAPI 3.0 schema
 
 ====== d) References:
@@ -802,7 +802,7 @@ Validate the Get Feature Operation Response.
     - To the Feature Collection which contains this Feature
 
     - Alternate encodings of this document in every other media type as identified by the compliance classes for this server.
-    
+
 . Validate that all links include the rel and type link parameters.
 
 ====== d) References:

core/standard/clause_7_core.adoc

@@ -389,6 +389,10 @@ in OpenAPI 3.0?
 
 include::requirements/core/REQ_fc-md-extent.adoc[]
 
+include::requirements/core/REC_fc-md-extent-single.adoc[]
+
+include::requirements/core/PER_fc-md-extent-extensions.adoc[]
+
 .link:https://raw.githubusercontent.com/opengeospatial/WFS_FES/master/core/openapi/schemas/collectionInfo.yaml[Schema for the metadata about a feature collection]
 [source,YAML]
 ----
@@ -409,9 +413,109 @@ properties:
   links:
     type: array
     items:
-      $ref: https://raw.githubusercontent.com/opengeospatial/WFS_FES/master/core/openapi/schemas/link.yaml
+      type: object
+      required:
+        - href
+      properties:
+        href:
+          type: string
+          example: http://data.example.com/buildings/123
+        rel:
+          type: string
+          example: alternate
+        type:
+          type: string
+          example: application/geo+json
+        hreflang:
+          type: string
+          example: en
+        title:
+          type: string
+          example: Trierer Strasse 70, 53115 Bonn
   extent:
-    $ref: https://raw.githubusercontent.com/opengeospatial/WFS_FES/master/core/openapi/schemas/extent.yaml
+    description: >-
+      The extent of the features in the collection. In the Core only spatial and temporal
+      extents are specified. Extensions may add additional members to represent other
+      extents, for example, thermal or pressure ranges.
+    type: object
+    properties:
+      spatial:
+        description: >-
+          The spatial extent of the features in the collection.
+        type: object
+        properties:
+          bbox:
+            description: >-
+              One or more bounding boxes that describe the spatial extent of the dataset.
+              In the Core only a single bounding box is supported. Extensions may support
+              additional areas. If multiple areas are provided, the union of the bounding
+              boxes describes the spatial extent.
+            type: array
+            minItems: 1
+            items:
+              description: >-
+                West, south, east, north edges of the bounding box. The coordinates
+                are in the coordinate reference system specified in `crs`. By default
+                this is WGS84 longitude/latitude.
+              type: array
+              minItems: 4
+              maxItems: 6
+              items:
+                type: number
+              example:
+                - -180
+                - -90
+                - 180
+                - 90
+          crs:
+            description: >-
+              Coordinate reference system of the coordinates in the spatial extent
+              (property `bbox`). The default reference system is WGS84 longitude/latitude.
+              In the Core this is the only supported coordinate reference system.
+              Extensions may support additional coordinate reference systems and add
+              additional enum values.
+            type: string
+            enum:
+              - 'http://www.opengis.net/def/crs/OGC/1.3/CRS84'
+            default: 'http://www.opengis.net/def/crs/OGC/1.3/CRS84'
+      temporal:
+        description: >-
+          The temporal extent of the features in the collection.
+        type: object
+        properties:
+          bbox:
+            description: >-
+              One or more time intervals that describe the temporal extent of the dataset.
+              In the Core only a single time interval is supported. Extensions may support
+              multiple intervals. If multiple intervals are provided, the union of the
+              intervals describes the temporal extent.
+            type: array
+            minItems: 1
+            items:
+              description: >-
+                Begin and end times of the time interval. The timestamps
+                are in the coordinate reference system specified in `trs`. By default
+                this is the Gregorian calendar.
+              type: array
+              minItems: 2
+              maxItems: 2
+              items:
+                type: string
+                format: date-time
+              example:
+                - '2011-11-11T12:22:11Z'
+                - '2012-11-24T12:32:43Z'
+          trs:
+            description: >-
+              Coordinate reference system of the coordinates in the temporal extent
+              (property `interval`). The default reference system is the Gregorian calendar.
+              In the Core this is the only supported temporal reference system.
+              Extensions may support additional temporal reference systems and add
+              additional enum values.
+            type: string
+            enum:
+              - 'http://www.opengis.net/def/uom/ISO-8601/0/Gregorian'
+            default: 'http://www.opengis.net/def/uom/ISO-8601/0/Gregorian'
   crs:
     description: the list of coordinate reference systems supported by the service; the first item is the default coordinate reference system
     type: array
@@ -421,7 +525,7 @@ properties:
       - http://www.opengis.net/def/crs/OGC/1.3/CRS84
 ----
 
-NOTE: The `crs` property is not used by this conformance class, but reserved for future use.
+NOTE: The `crs` property of the `collectionInfo` object is not used by this conformance class, but reserved for future use.
 
 .Feature collection metadata response document
 =================
@@ -434,8 +538,9 @@ Representations of the metadata resource in other formats are referenced using l
 Additional links to a GML application schema for the building data and to
 a web page that has additional information about buildings are provided using link:https://www.iana.org/assignments/link-relations/link-relations.xhtml[link relation type] "describedBy".
 
-Coordinate reference system information is not provided as the service provides
-geometries only in the default system (WGS84 longitude/latitude).
+Reference system information is not provided as the service provides geometries
+only in the default systems (spatial: WGS84 longitude/latitude; temporal:
+Gregorian calendar).
 
 [source,JSON]
 ----
@@ -454,8 +559,12 @@ geometries only in the default system (WGS84 longitude/latitude).
       "title": "Buildings",
       "description": "Buildings in the city of Bonn.",
       "extent": {
-        "spatial": [ 7.01, 50.63, 7.22, 50.78 ],
-        "temporal": [ "2010-02-15T12:34:56Z", "2018-03-18T12:11:00Z" ]
+        "spatial": {
+          "bbox": [ [ 7.01, 50.63, 7.22, 50.78 ] ]
+        },
+        "temporal": {
+          "interval": [ [ "2010-02-15T12:34:56Z", "2018-03-18T12:11:00Z" ] ]
+        }
       },
       "links": [
         { "href": "http://data.example.org/collections/buildings/items",

core/standard/requirements/core/PER_fc-md-extent-extensions.adoc

@@ -0,0 +1,14 @@
+[width="90%",cols="2,6a"]
+|===
+|Permission {counter:per-id} |/per/core/fc-md-extent-extensions +
+
+The Core only specifies requirements for spatial and temporal extents.
+However, the `extent` object MAY be extended with additional members
+to represent other extents, for example, thermal or pressure ranges.
+
+The Core only supports spatial extents in WGS84 longitude/latitude and
+temporal extents in the Gregorian calendar (these are the only enum values in
+link:https://raw.githubusercontent.com/opengeospatial/WFS_FES/master/core/openapi/schemas/[extent.yaml]).
+Extension to the Core MAY add additional reference systems to the
+`extent` object.
+|===

core/standard/requirements/core/REC_fc-md-extent-single.adoc

@@ -0,0 +1,12 @@
+[[fc_md_extent_single]]
+[width="90%",cols="2,6a"]
+|===
+|Recommendation {counter:rec-id} |/rec/core/fc-md-extent-single +
+
+While the spatial and temporal extents support multiple bounding boxes
+(`bbox` array) and time intervals (`interval` array) for advanced use cases,
+implementations SHOULD provide only a single bounding box or time interval
+unless the use of multiple values is important for the use of the dataset
+and agents using the API are known to be support multiple bounding boxes
+or time intervals.
+|===

core/standard/requirements/core/REQ_fc-md-extent.adoc

@@ -2,7 +2,7 @@
 |===
 |*Requirement {counter:req-id}* |/req/core/fc-md-extent +
 
-For each feature collection, the `extent` property, if provided, SHALL be a bounding box that includes all spatial and temporal geometries in this collection.
+For each feature collection, the `extent` property, if provided, SHALL provide bounding boxes that include all spatial geometries and time intervals that include all temporal geometries in this collection.
 
 If a feature has multiple properties with spatial or temporal information, it is the decision of the server whether only a single spatial or temporal geometry property is used to determine the extent or all relevant geometries.
 |===