Replace Boston dataset from examples and tests.

[desilinguist]

This PR closes #700.

Replace boston dataset with california dataset everywhere.

• Remove boston example and replace with new california example.
• Remove make_boston_example_data.py script and add make_california_example_data.py script instead and sub-sample the full dataset is huge.
• Remove references to boston dataset from everywhere.
• Remove boston example test data and add california example test data.
• Replace boston name in sample test data.

[codecov]

Codecov Report

Base: 96.90% // Head: 96.90% // No change to project coverage 👍

Coverage data is based on head (8f5b6f7) compared to base (c49147b).
Patch coverage: 22.22% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #717   +/-   ##
=======================================
  Coverage   96.90%   96.90%           
=======================================
  Files          63       63           
  Lines        9271     9271           
=======================================
  Hits         8984     8984           
  Misses        287      287           

Impacted Files	Coverage Δ
examples/make_california_example_data.py	0.00% <0.00%> (ø)
skll/experiments/output.py	97.38% <ø> (ø)
tests/test_regression.py	99.64% <ø> (ø)
tests/test_examples.py	98.36% <100.00%> (ø)

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