added RNA.CDS_count() and RNA.exon_count() methods
Made Gene and RNA objects hashable
Added Gene.mRNA_count() and Gene.remove_mRNA()
Non-functional version of select_training_and_evaluation_transcripts.py, storing just so we can see why this doesn't work later
Another version that still prints duplicate mRNAs - this time using list comprehension
Now works, but --retain_composition not yet functional
Added --output_id_list option