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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
No, wait, I misunderstood your question.  (1) Yes, this is the adamLoad fn from the ADAMContext.  (2) Yes, this won't load Features from a GTF file, it'll only load them from a parquet file.  

There's a missing converter in here, somewhere...