auto_regression: return covariance

[mathause]

• Closes auto_regression: return co-variance instead of standard deviation #306
• Tests updated
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

Despite #307 I want to get this in. As always unsure if that should be cov or covariance.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (acd80e4) 87.00% compared to head (d90b1b3) 87.03%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   87.00%   87.03%   +0.02%     
==========================================
  Files          38       38              
  Lines        1601     1604       +3     
==========================================
+ Hits         1393     1396       +3     
  Misses        208      208              

Flag	Coverage Δ
unittests	87.03% <100.00%> (+0.02%)	⬆️

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Files	Coverage Δ
mesmer/calibrate_mesmer/train_gv.py	95.00% <100.00%> (+0.08%)	⬆️
mesmer/calibrate_mesmer/train_lv.py	84.33% <100.00%> (+0.38%)	⬆️
mesmer/stats/auto_regression.py	100.00% <100.00%> (ø)

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