Add aws

Added allele freq calc function eg

```
import anhima
import scipy.stats
import numpy as np

# simulate non-uniform variant positions
n_variants = 1000
p = 0
pos = [p]
for i in range(n_variants-1):
    gap = int(np.abs(np.cos(i/100))*100)
    p += gap
    pos.append(p)
pos = np.array(pos)

# simulate genotypes 
n_samples = 100
ploidy = 2
af_dist = scipy.stats.beta(a=.4, b=.6)
p_missing = 0

population_a = anhima.sim.simulate_biallelic_genotypes(n_variants, 10, af_dist, missing, ploidy)
population_b = anhima.sim.simulate_biallelic_genotypes(n_variants, 5, af_dist, missing, ploidy)
population_c = anhima.sim.simulate_biallelic_genotypes(n_variants, 5, af_dist, missing, ploidy)

calculate_aws(population_a, population_b, population_c)
```