add calc_gaspari_cohn_correlation_matrices

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

Add function to calculate Gaspari-Cohn correlation matrices for a range of localisation radii. Again, blatantly stolen from #109, so @znicholls should really get the credit here. I pass geodist instead of lon, lat because I still intend to do #62 and for this it is much easier if we pass the geodist directly. Makes it also a bit more explicit what is happening.

The tests are crap - they repeat the code in the function, which renders almost worthless. Still it checks that it is run.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02% 🎉

Comparison is base (1358e63) 86.89% compared to head (0dd31ef) 86.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
+ Coverage   86.89%   86.92%   +0.02%     
==========================================
  Files          38       38              
  Lines        1580     1583       +3     
==========================================
+ Hits         1373     1376       +3     
  Misses        207      207              

Flag	Coverage Δ
unittests	86.92% <100.00%> (+0.02%)	⬆️

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Files Changed	Coverage Δ
mesmer/core/computation.py	100.00% <100.00%> (ø)

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[mathause]

I'm ignoring the faulty rtd built. I think it's a fluke.