Script to combine genome builds

[davmlaw]

It might be worth combining GRCh37/GRCh38 into a single transcript - some things remain the same (ie start/stop codon and gene symbol etc) the build specific things can be in a dict.

HGVS makes a query asking what builds/contigs are available - if we keep them separate, this is 2 API calls rather than 1

If we store things per contig, we could save a tiny amount of space with MT transcripts. Might just be easier to store in genome builds as other users would likely want that