[patch:lib] Fix major kNN label scoring bug

[eonu]

Fixes #186 and generally improves the code in the KNNClassifier.

• Split _find_nearest into two functions:

  • _find_k_nearest: Finds the label and weighting/score of the k nearest neighbors with numpy.argpartition.
  • _find_max_labels: Finds the labels out of the nearest k which had the highest total score.

  A major bug in finding the labels with the highest total score was spotted by @manisci (thanks!). This has now been fixed, and splitting the function into the two described above made it easier to write unit tests for kNN.

  Release notes for older versions will also be updated to include a warning about this bug.

• Disabled progress bars for multi-processed kNN predictions due to rendering issues.

• Added a _predict function for the logic of making a single prediction, and updated _chunk_predict to call _predict for each example.

• Renamed _argmax to _multi_argmax for clarity.