Support RefGet

[jdidion]

When the primary file is not available or does not contain contig information, use RefGet to fetch the information on contig names and sizes, rather than requiring a .genome file. If the primary file is supplied and it specified the ID or hash of the reference genome, use that to lookup the metadata; otherwise, require the genome ID or hash to be passed as a command line option.

RefGet spec: http://samtools.github.io/hts-specs/refget.html

Server list:

• https://www.ebi.ac.uk/ena/cram/

Python client: https://github.com/ga4gh/refget-client