linear regression: add xarray wrapper

[mathause]

• Closes xarray wrapper for linear_regression #119
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

[codecov-commenter]

Codecov Report

Merging #123 (dbe4ecf) into master (54d4406) will increase coverage by 0.54%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   78.61%   79.16%   +0.54%     
==========================================
  Files          27       28       +1     
  Lines        1300     1339      +39     
==========================================
+ Hits         1022     1060      +38     
- Misses        278      279       +1     

Flag	Coverage Δ
unittests	79.16% <97.50%> (+0.54%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mesmer/core/utils.py	94.11% <94.11%> (ø)
mesmer/core/linear_regression.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 54d4406...dbe4ecf. Read the comment docs.

[mathause]

I am not super happy with this because:

• testing the sanity of 4 input DataArray inputs harms my sanity
• (I could use mock but not sure how much better this is)
• this should probably be encapsulated in a class?

class LinearRegression:don't really like

    def fit(predictors, target, dim, weights):
        pass

    def predict(predictors, dim):
        pass

    def residuals():
        pass

[znicholls]

lgtm

[mathause]

I am merging this & move the tests/utils.py in another PR.