Dividing by zero should be NaN

[lkirk]

Allow C and Python portions of the code to divide by zero natively, relying on their respective runtime implementations. Remove code that forces the two-locus statistics to return zero when we encounter 0/0 situations. In testing, it was ensured that some test cases return NaN values, which we diff appropriately.

Fixes #2907

PR Checklist:

• [X] Tests that fully cover new/changed functionality.
• [ ] Documentation including tutorial content if appropriate.
• [ ] Changelogs, if there are API changes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7a0b863) 89.75% compared to head (e40ba27) 89.75%.

Additional details and impacted files

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[lkirk]

Squashed and ready.