use LinearRegression in train_gt_ic_OLSVOLC

[mathause]

After #144 we can now use mesmer.core.linear_regression.LinearRegression in train_gt_ic_OLSVOLC.

• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

[codecov-commenter]

Codecov Report

Merging #145 (b953704) into master (5944564) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   79.25%   79.19%   -0.06%     
==========================================
  Files          30       30              
  Lines        1393     1389       -4     
==========================================
- Hits         1104     1100       -4     
  Misses        289      289              

Flag	Coverage Δ
unittests	79.19% <100.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
mesmer/calibrate_mesmer/train_gt.py	81.60% <100.00%> (ø)
mesmer/io/load_obs.py	27.08% <100.00%> (-5.61%)	⬇️

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[znicholls]

lgtm

[mathause]

Thanks for the review!