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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
"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." <-- yeah, and I think this is the way it's handled in the GBFF files you get from Refseq, too... 

So, again, though: what should the Gene/Transcript/Exon classes look like (should CDS be attached to the transcript as a single region? Or should it be multiple regions as it is here? Should we differentiate between 5' and 3' UTRs? should we model the locations of codons as the above GTF clearly does?  any other questions?) independent of what's in the different formatted files...?