Two locus user-facing python ld matrix API

[lkirk]

Description

This is an implementation of the user-facing python API for ld_matrix as discussed in #2829

I've added some tests that cover most edge cases. I think I'd like to add some tests that store literal matricies, but I wanted to get eyes on this before I added those final tests.

PR Checklist:

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

[lkirk]

cc @apragsdale

[codecov[bot]]

Codecov Report

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

Comparison is base (04e04aa) 89.73% compared to head (2caf278) 89.75%.

Additional details and impacted files

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[lkirk]

re: docstrings, there's already one present in the code for r2, so I left that one in place but got rid of all of the ones in this PR.

Input validation testing is done, all testable branches are covered.

I'll open an issue for further NaN discussions.

[lkirk]

I'm a little confused by the results of the coverage suite. It's annotating lines on the diff, but the patch coverage is reported as 100%.

I've opened #2907 to further investigate the NaN behavior.

Squashed down preemptively, but let me know if there's anything else needed here.