PBS API

[daletovar]

I wanted to spark some discussion on what the api for doing pbs should look like. Based on this notebook that @alimanfoo made, it looks like we'll need to implement something like this:

1. an allele count for each population.
   • sgkit.count_alleles will do this.
2. locate and subset to segregating variants at sufficient frequency.
   • requires an is_biallelic_01 function, or something equivalent.
   • requires a to_frequencies function, or some equivalent operation.
3. setup windows over the genome.
   • requires a moving_statistic function.
4. calculate the PBS in a moving window.
   • requires a pbs function.

[jeromekelleher]

Closing in favour of #230.