+ momentarily work off a forked cyvcf2
+ momentarily work off a forked cyvcf2

  - need to use a version of cyvcf2 that has the following fix:

    https://github.com/brentp/cyvcf2/pull/55

    Otherwise, the allele balance calculations will be incorrect.

indraniel/yaps2,indraniel/yaps2,indraniel/yaps2