Implement haplotype counts in haplo-stats module

[alexlancaster]

haplo-stats is not reporting haplotype counts (splitting off as a separate issue from #38 )

cc: @sjmack

[alexlancaster]

A quick review of the code suggests that the haplotype count generation in haplostats was disabled in the XML output pending a check of the numbers, there's a FIXME:

https://github.com/alexlancaster/pypop/blob/main/src/PyPop/Haplo.py#L1013

To test this, we can uncomment the code and check the results manually against a known set, and include a corresponding unit test.