gaspari_cohn: allow passing xr.DataArray

[mathause]

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

Allow passing xr.DataArray to gaspari_cohn. Maybe this needs a check such that no Dataset is passed? - Done.

This was motivated by looking at #109 again.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (bcbf5c9) 86.73% compared to head (76d9914) 86.79%.

❗ Current head 76d9914 differs from pull request most recent head f96579f. Consider uploading reports for the commit f96579f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
+ Coverage   86.73%   86.79%   +0.05%     
==========================================
  Files          38       38              
  Lines        1561     1568       +7     
==========================================
+ Hits         1354     1361       +7     
  Misses        207      207              

Flag	Coverage Δ
unittests	86.79% <100.00%> (+0.05%)	⬆️

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

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[mathause]

Obviously it does not matter a bit but I like the solution I found for supporting numpy arrays and xr.DataArray. I don't think computation is a very good location for the function but I could not come up with a good name now (stats.kernels? - but is it a kernel?, stats.covariance sounds strange).