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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
Yeah.  That's why I'm writing it as a flatMap, but expressing my understanding in a comment.  

I don't think that Ensembl or Gencode or Refseq model shared exons, but it's certainly something that someone *could* model.