Extents not easily extensible as the connections to the reference systems are not clear #168

[cportele]

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"
    }
  }
}