Fix label ordering in binary logistic regression

[espenloov]

Fixes #393. It now assigns positive/negative labels based on Ord ordering of the label values rather than encounter order in the training data.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.73%. Comparing base (c7c2af5) to head (6dcc5cf).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #432      +/-   ##
==========================================
- Coverage   76.76%   76.73%   -0.04%     
==========================================
  Files         106      106              
  Lines        7411     7410       -1     
==========================================
- Hits         5689     5686       -3     
- Misses       1722     1724       +2     

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[relf]

Thanks for the fix!