Extract allele frequency data from 1000G VCFs

[grosscol]

Create a new workflow for allelic frequency information. The current AF data comes along with the VEP process due to the --af flag.

The allelic frequency information from the VEP output appears to be incomplete. E.g. 1-55063514-G-A should have AF data, but it does not appear to be present.

1. Download VCFs for 1000G on GRCh38 into reference data storage: https://www.internationalgenome.org/data-portal/data-collection/grch38
2. Extract AF and *_AF fields. SNV ids are pos-ref-alt
3. Convert to Mongo's bson format for use with mongoimport.