[ADAM-327] Adding Gene, Transcript, and Exon models

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
Maybe that's true, but that also adds a bit of complication. If you wanted to add another level to the hierarchy, you'd need to have exonIds to group the lower level objects by.  What are all the possible "subtypes" of exon?  Another way could be to model exons like in GTF, and have a separate, single, CDS feature that covers some region of the spliced transcript.  All it would really define would be the boundary between the UTRs and the coding areas.