Update type fields to use union types / interfaces

[alancleary]

Currently the GraphQL types are basically a verbatim implementation of the InterMine types. This means certain types contain fields that are mutually exclusive. For example, the Trait type can have a value for its qtlStudy field or its gwas field, but not both. The canonical way to handle this in GraphQL is to use a single field with a union or interface as its type. This will make the semantics of our GraphQL API more obvious and simplify client code that uses these types while enabling canonical functionality, e.g. filtering queries/results by type.

<edit>
Commit 24e8445 adds the necessary resolver support.
</edit>

The following types should be updated to use this approach:

• Location: chromsome/supercontig -> locatedOn
• Trait: qtlStudy/gwas -> study; remove qlts and gwasResults since they can be accessed via study
• DataSet: bioEntities; already generalized to type [BioEntity!]! but not implemented correctly
• SequenceFeature:
  • childFeatures and overlappingFeatures; both fields are already generalized to type [SequenceFeature!]! but are not implemented correctly
  • chromsomeLocation/supercontigLocation -> location
  • chromsome and supercontig can be removed because they will be accessible via location.locatedOn
• ExpressionValue: feature; already generalized to type SequenceFeature but not implemented correctly
• PanGeneSets: transcripts; already generalized to type [Transcript!]! but not implemented correctly