import gnomad data

[jjgao]

Currently gnomad data is being pulled from myvariant.info. Importing gnomad import GN will remove the dependency to myvariant.info as a external resource.

• download from https://gnomad.broadinstitute.org/downloads
  • v2 for grch37
  • v3 for grch38
• filter to ancestry allele frequencies that we use, e.g. https://www.genomenexus.org/variant/17:g.41276045_41276046del
  
• add the data to importer module
• import to mongo
• develop api endpoint