Documentation and tests for LD matrix output dimensions

[apragsdale]

Noted by @petrelharp (in #3179) the immediate next steps for moving the LD methods forward are:

• write docs explaining output dimensions
• explicitly test output dimensions (the current test just tests the number of dimensions, not that they're in the right order)

For the first, docs will need to be written generally for the LD matrix methods, and special attention to output dimension behavior will be needed.

cc: @lkirk

[petrelharp]

Note those are some of the next steps, I don't know if they're the next steps.

[lkirk]

When we last met, we had discussed:

• explicitly testing output dimensions. I suppose I misinterpreted the goal. As mentioned, I'm testing the number of dimensions and not the actual values of the dimensions. I can include this in the next PR (c/Python).
• PR for the c/user facing Python API (specifically the multi population changes)
• PR for documentation, which would mark the official release of the feature. This will include 1) API docs (in the form of docstrings) 2) a more thorough treatment in the tskit stats docs 3) some simple naive tests that are around a dozen lines, based on the genotype matrix (for more explicit stats testing/documentation).

This is my understanding of the work needed to complete the LD matrix methods