Fix confusion matrix

[relf]

This PR supersedes PR #249 taking into account review comments

• Use predicted and ground truth labels to compute confusion matrix
• Make confusion matrix reproducible by sorting classes (labels). Fixes the order of classification #341.
• Use sensible default for binary confusion matrix

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 35.49%. Comparing base (9744625) to head (eed0c49).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/metrics_classification.rs	50.00%	2 Missing ⚠️
src/dataset/mod.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   35.44%   35.49%   +0.04%     
==========================================
  Files          96       96              
  Lines        6371     6379       +8     
==========================================
+ Hits         2258     2264       +6     
- Misses       4113     4115       +2     

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