Incorporate the concept of a "schema collection" into the standard

[perrygreenfield]

It is useful to have an overarching version that ties together a group of related schemas – for example, software can read a list of schemas associated with that version and write objects to an ASDF file that validate against schemas in that particular set. For the ASDF core schemas, the ASDF Standard version provides that overarching version. For user-defined schemas, there is currently no solution, and no library support for selecting a particular version of user-defined schema on write. Define a format for a schema collection manifest

This will be a YAML file, analogous to the existing version_map-x.y.z.yaml files, that defines a schema collection version. The file will need the following features:

• Unique id that defines the name and version
  • Could be a similar URI to the schema ids, for example
    http://stsci.edu/schema_collections/core-1.0.0.yaml
    • Would be used by implementations to allow users to select a particular version of a
      schema collection
  • List of schemas in the "collection"
    • A list of schema id URIs