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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
After going through all this (and yes, you're right, I think with teh current setup you'd need to encode the filetype in the parquet file somewhere) I kinda think I'm leaning towards ditching the feature hierarchy stuff altogether, maybe make those part of the parsing framework, but really focus on having the Feature schema contain all the necessary fields in a single, unified format.  

Thoughts?