Add Wattersons_theta

[szhan]

Fixes #1522

[codecov[bot]]

Codecov Report

Merging #2368 (f0d6387) into main (c12c160) will decrease coverage by 12.36%. The diff coverage is 25.00%.

@@             Coverage Diff             @@
##             main    #2368       +/-   ##
===========================================
- Coverage   93.34%   80.98%   -12.37%     
===========================================
  Files          28       28               
  Lines       27006    26763      -243     
  Branches     1253     1244        -9     
===========================================
- Hits        25209    21674     -3535     
- Misses       1763     5015     +3252     
- Partials       34       74       +40     

Flag	Coverage Δ
c-tests	92.26% <ø> (ø)
lwt-tests	89.05% <ø> (ø)
python-c-tests	70.89% <25.00%> (-0.02%)	:arrow_down:
python-tests	?

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Impacted Files	Coverage Δ
python/tskit/trees.py	44.91% <25.00%> (-53.75%)	:arrow_down:
python/tskit/cli.py	0.00% <0.00%> (-96.19%)	:arrow_down:
python/tskit/vcf.py	7.75% <0.00%> (-90.52%)	:arrow_down:
python/tskit/text_formats.py	9.87% <0.00%> (-90.13%)	:arrow_down:
python/tskit/drawing.py	11.02% <0.00%> (-87.97%)	:arrow_down:
python/tskit/combinatorics.py	14.26% <0.00%> (-85.11%)	:arrow_down:
python/tskit/genotypes.py	33.75% <0.00%> (-65.14%)	:arrow_down:
python/tskit/stats.py	35.13% <0.00%> (-64.87%)	:arrow_down:
python/tskit/util.py	60.42% <0.00%> (-39.58%)	:arrow_down:
... and 3 more

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