xarray wrapper to draw auto-regression samples

[mathause]

• Closes auto-regression: xarray wrapper for predict #151
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

A lot is still missing (tests, docs). But as a first draft.

[mathause]

@yquilcaille can you read the docstrings and let me know if this makes sense?

I added a leading underscore to the name as I am not yet sure I'll expose the functions as is, but that can be changed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (69cb102) 87.16% compared to head (ab49389) 87.57%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
+ Coverage   87.16%   87.57%   +0.41%     
==========================================
  Files          38       38              
  Lines        1620     1674      +54     
==========================================
+ Hits         1412     1466      +54     
  Misses        208      208              

Flag	Coverage Δ
unittests	87.57% <100.00%> (+0.41%)	⬆️

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Files	Coverage Δ
mesmer/create_emulations/create_emus_gv.py	89.18% <100.00%> (+1.68%)	⬆️
mesmer/create_emulations/create_emus_lv.py	93.44% <100.00%> (+0.98%)	⬆️
mesmer/stats/auto_regression.py	100.00% <100.00%> (ø)

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