Update Coverages to reflect Common `envelope` as part of `extent` property dimensions

[jerstlouis]

We need to update the standard to reflect the resolution of this issue in Common:

https://github.com/opengeospatial/oapi_common/issues/91#issuecomment-720466625

Discussions on single vs. multiple bounding box / time intervals led to adding a new envelope member of each dimension for which there will always be only one to make it easier for clients who only support a single extent, without having to calculate a union themselves. In coverages, the multiple bbox or intervals should be optional, while the envelope should be required?

NOTE: This belongs inside the extent property of the collection description resource.

[jerstlouis]

Proposed to Common that axes be named explicitly for envelope:

https://github.com/opengeospatial/oapi_common/issues/91#issuecomment-721550163

Tentative changes:

https://github.com/jerstlouis/ogc_api_coverages/blob/collectionEnvelope/standard/examples/JSON/collection_info_example.json

[jerstlouis]

The updated unified schema for extent proposed for Coverages is now available at:.

https://github.com/opengeospatial/ogcapi-coverages/blob/schema-work/openapi/schemas/coverages-core/envelope.yaml

I believe it is mostly compatible with Common, Features as well as EDR (except for the additional restriction on what additional properties should be).

[jerstlouis]

2021-07-28: The schemas from schema-work were merged, and it includes the proposed extension to OGC API - Common - Part 2 schemas.