Add aaf() option to --gt-filter

[arq5x]

It would be nice to have the ability to filter variants based on the alternate allele frequency in specific subsets of individuals. Currently, we can filter based on the counts of specific genotypes, but aaf would be more generalizable.

For example:

(phenotype==2).(aaf < 0.01)

[brentp]

#584 would enable this.

[arq5x]

Isn't that for the allele frequency in a single sample? What I meant to articulate was filtering based on the genotyped AF across all individuals meeting a specific phenotype.