lin reg: add exclude to predict

[mathause]

• Closes allow skipping predictors in LinearRegression.predict #321
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

I need to think a bit more about this - I wrote this so we can split up the linear regression. However, this will include the intercept twice. So I may have to add the possibility to add the intercept in exclude.

if "intercept" in exclude:
    prediction = xr.zeros_like(params.intercept)
else:
    prediction = params.intercept

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (67a7194) 87.88% compared to head (f31247f) 87.89%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #354   +/-   ##
=======================================
  Coverage   87.88%   87.89%           
=======================================
  Files          40       40           
  Lines        1742     1743    +1     
=======================================
+ Hits         1531     1532    +1     
  Misses        211      211           

Flag	Coverage Δ
unittests	87.89% <100.00%> (+<0.01%)	⬆️

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