Integrate GA4GH Use Case review from EBI

[kozbo]

The EBI team is working on a review of use cases for the GA4GH API. They are compiling a document on a fork of schemas and they will merge it in after review.

Note that there are a number of issues already here in schemas to track use cases so there may be some clean up once we merge. The goals it to have each use case result in an issue. If the use case is already satisfied then it can be closed with a detailed description of how the case can be met with the current API.

Here are some pre-existing issues around use cases from the issue backlog. Note that some are just place holders:
#788 Federated Query Use Cases
#446 Metadata use cases
#226 Variant Annotation API - use cases
#429 Cancer virus and human decoy search use cases not supported
#256 Human-readable documentation / explicit use cases missing -- (this one strikes me as a lot of talk about the need for use cases, without a clear resolution)
(and probably more that I haven't found yet)

Also we have these docs:
https://github.com/ga4gh/schemas/wiki/Use-case-discussion-board
https://github.com/ga4gh/schemas/wiki/Use-cases-for-discovery-of-data
https://github.com/ga4gh/schemas/blob/62111f8430b3857ae4fade759fd3edc808607a5b/doc/source/api/rna_quantifications.rst (6 RNA use cases here)

It will be good to harmonize this to have the use cases in one location!