openapi/schemas: Applied fixes for issues identified by Clemens

- This addresses the last 3 bullet points of opengeospatial/ogcapi-common#91 (comment)
- extent.yaml, coverageInfo.yaml, envelope.yaml modified

standard/openapi/schemas/common-geodata/extent.yaml

@@ -51,10 +51,12 @@ properties:
       interval:
         description: |-
           One or more time intervals that describe the temporal extent of the dataset.
-          The value `null` is supported and indicates an open time intervall.
-          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.
+
+          The value `null` is supported and indicates an unbounded or half-bounded interval.
+          The first time interval describes the overall temporal extent of the data.
+          All subsequent time intervals describe more precise time intervals, e.g., to identify clusters of data.
+          Clients only interested in the overall extent will only need
+          to access the first item (a pair of lower and upper bound values).
         type: array
         minItems: 1
         items:

standard/openapi/schemas/coverages-core/coverageInfo.yaml

@@ -6,6 +6,12 @@ allOf:
     properties:
       extent:
         $ref: 'envelope.yaml'
+      orderedAxes:
+        type: array
+        minItems: 1
+        items:
+           type: string
+           description: axis labels of all dimensions listed in CRS order. Spatial dimensions should be listed separately here.
       domainSet:
         allOf:
           - $ref: '../cis/domainSet.yaml'

standard/openapi/schemas/coverages-core/envelope.yaml

@@ -4,13 +4,6 @@ description: |-
 allOf:
   - $ref: '../common-geodata/extent.yaml'
   - type: object
-    properties:
-       - orderedAxes:
-            type: array
-            minItems: 1
-            items:
-               type: string
-               description: axis labels of all dimensions listed in CRS order. Spatial dimensions should be listed separately here.
     additionalProperties:
        description: |-
          The domain intervals for any additional dimensions of the extent (envelope) beyond those described in temporal and spatial.
@@ -20,10 +13,12 @@ allOf:
             - interval:
               description: |-
                 One or more intervals that describe the extent for this dimension of the dataset.
-                The value `null` is supported and indicates an open interval.
-                In the Core only a single interval is supported. Extensions may support
-                multiple intervals. If multiple intervals are provided, the union of the
-                intervals describes the overall extent for the dimension.
+
+                The value `null` is supported and indicates an unbounded or half-bounded interval.
+                The first interval describes the overall extent of the data for this dimension.
+                All subsequent intervals describe more precise intervals, e.g., to identify clusters of data.
+                Clients only interested in the overall extent will only need
+                to access the first item (a pair of lower and upper bound values).
               type: array
               minItems: 1
               items:
@@ -34,8 +29,9 @@ allOf:
                 minItems: 2
                 maxItems: 2
                 items:
-                  type: string
-                  nullable: true
+                  - type: string
+                  - type: number
+                  - type: null
                 example:
                   - '2011-11-11T12:22:11Z'
                   - 32.5