Add separate function for Watterson's theta

[percyfal]

sgkit-allel has functionality to calculate Watterson's theta but in sgkit it is calculated implicitly in Tajima's D function. For some applications direct access to Watterson's theta would be of practical use.

[percyfal]

I could draft a PR since I think this is a pretty straightforward code refactoring operation.

[jeromekelleher]

That would be great, thanks @percyfal !

[milesroberts-123]

@percyfal Was this ever accomplished? I was thinking of attempting to add a Watterson's theta function if no one else was