GFF3 parser ignoring some features.

[ejacox]

If a feature has multiple parents, the gff3 parser only takes the first one. It should create features for all parents.

[ejacox]

My mistake. It seems to work correctly. There should be a test for this, though. A gff test file with a feature with multiple parents is needed.

[david4096]

We're currently covering gff3 parsing code implicitly as part of the ETL process for these data. By splitting the datamodel into components, I hope to isolate the coverage of these tests outside of the server.