[WIP] User Story PBS

[daletovar]

After doing some initial data preparation steps, PBS consists of doing:

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.
   -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.

Here is the notebook that @alimanfoo made.

[jerowe]

@daletovar - https://github.com/pystatgen/sgkit/issues/29