Move partitioning conformance classes to Part 1 & add subset query parameter

[jerstlouis]

Suggesting that:

• bbox,
• subset,
• map tiles

be separate conformance classes, but all defined in Part 1 (they are currently in https://github.com/opengeospatial/ogcapi-maps/tree/master/extensions/partitioning/standard).

This would reflect the fact that bbox is a basic capability that most implementations will want to implement, yet allow implementers to achieve conformance with Core, and only implement the partitioning mechanisms that they care to support initially.

[joanma747]

We accepted this proposal in the OGC MM 120.

[joanma747]

WE should implement this in the document structure in github.

[cmheazel]

There are some API Modules at API-Common that you may find useful. Look under the API_Modules directory.