Fix confusion matrix using only predictions as source for labels

[levkk]

Fix confusing matrix incorrectly using labels from predict only instead of using labels from predict and ground truth. Ideally we should expose the Scikit-like API that passes in all the labels, in case the labels in the test set are not all inclusive (which would be a mistake in train/test partitioning, but can happen).

I'm somewhat confused by the way the API is written because the argument for the confusion_matrix method is called ground_truth, but shouldn't it be the predicted points instead?

[codecov-commenter]

Codecov Report

Base: 39.24% // Head: 39.26% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (3356d42) compared to base (5ebe23c). Patch coverage: 60.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   39.24%   39.26%   +0.02%     
==========================================
  Files          92       92              
  Lines        6085     6089       +4     
==========================================
+ Hits         2388     2391       +3     
- Misses       3697     3698       +1     

Impacted Files	Coverage Δ
src/dataset/mod.rs	29.03% <50.00%> (-0.60%)	:arrow_down:
src/metrics_classification.rs	38.36% <100.00%> (-0.63%)	:arrow_down:
algorithms/linfa-nn/src/linear.rs	45.16% <0.00%> (-1.72%)	:arrow_down:
src/correlation.rs	29.57% <0.00%> (-1.41%)	:arrow_down:
algorithms/linfa-svm/src/classification.rs	46.49% <0.00%> (-0.88%)	:arrow_down:
...rithms/linfa-trees/src/decision_trees/algorithm.rs	36.60% <0.00%> (-0.45%)	:arrow_down:
algorithms/linfa-nn/tests/nn.rs	78.04% <0.00%> (ø)
algorithms/linfa-linear/src/glm/mod.rs	52.77% <0.00%> (ø)
... and 3 more

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[YuhanLiin]

The argument is ground_truth because self is the predicted points. The point about using labels from both sources still stands though.