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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
It's not going to be a large sort compared to the size of the read set, no? I imagine that the sort would be a perf optimization over multiple joins + groupBys, but I could be wrong. In any case, this is a perf optimization. We should probably leave it as a TODO for later.