Replace contig with region in GWSS functions

[leehart]

Resolves #375

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[jonbrenas]

Maybe using a random region instead of a random contig in at least some of the tests would be better. The function random_region_str from tests/anoph/conftest.py exists and there are a few examples of use in test_snp_data.py, for instance.

[leehart]

Just to note that there are other places in the code unrelated to this PR where a random contig is used for a region in testing, e.g.

• test_biallelic_diplotype_pairwise_distance_with_metric
• test_njt_with_metric
• test_njt_with_algorithm
• test_plot_njt
• test_average_fst
• test_average_fst_with_min_cohort_size
• test_pca_plotting
• test_pca_exclude_samples
• test_pca_fit_exclude_samples
• test_plink_converter

There might be a valid reason for choosing a random contig over a random region string, I'm not sure, but we could deal with those cases in another PR, in needs be.

[leehart]

I'm getting IndexError: index 0 is out of bounds for axis 0 with size 0 in a few tests after switching to random region strings, so it seems worthwhile!