moving url construction into separate module, getting ready for multiple species
deleted stray char
exons can overlap multiple start and stop codons
added tests for locus offsets
moved dataframe access out of EnsemblRelease into GTF object, added skeleton of a ReferenceTranscripts object
parsing and indexing transcript references using pyfaidx
CDS transcript offsets and property for transcript completeness
renamed methods for getting unique values at a locus to column_values_at_locus and distinct_column_values_at_locus
more comments, test ID lengths for multiple releases and feature types
check start codon offsets are only for fully overlapping start codons
minor fix
merge and version bump