auto-detect reference genome

[pjvandehaar]

• Make detect-ref (host on pypi) which detects the compatible reference genomes for a set of variants.
  • http://hgdownload.cse.ucsc.edu/goldenPath/hg19/chromosomes/ provides per-chromosome per-build .fa.gzs, about ~50MB each. (Cache after downloading.)
  • it'd be easy enough to support vcfs, if that'd be useful.
• use it in pheweb's parser. then what? automatically liftover to GRCh38?