Get r2_array returns new array.

[jeromekelleher]

Currently the ld calculator get_r2_array method reuses the same memory between calls, leading to subtle bugs. Change this.

Closes #396

[jeromekelleher]

This hasn't been fully tested yet, needs some more low-level checking for memory leaks.

[jeromekelleher]

Note: might be simpler for now to just allocate a buffer for each call to get_r2_array, rather than messing with the C API. We're going to be replacing all of this at some point soon, hopefully, so not much point in messing with it.

[codecov]

Codecov Report

Merging #397 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
- Coverage   86.53%   86.52%   -0.01%     
==========================================
  Files          20       20              
  Lines       14081    14072       -9     
  Branches     2748     2750       +2     
==========================================
- Hits        12185    12176       -9     
  Misses        977      977              
  Partials      919      919

Flag	Coverage Δ
#c_tests	87.62% <100%> (-0.02%)	⬇️
#python_c_tests	90.38% <100%> (-0.02%)	⬇️
#python_tests	99.25% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
python/tskit/stats.py	100% <100%> (ø)	⬆️
python/tskit/trees.py	98.7% <0%> (-0.01%)	⬇️

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