Allow internal plugins to register parametric types
Fix output type of learn_regressor() function
Make Classifier a parametric type
Add support for string labels to ML plugin
Add serialization id test for unit normalizer
Fix IntelliJ warnings in MLFunctions
Generalize evaluate_classifier_predictions

It now works with multi-class classifiers and also with string labels
Remove unnecessary synchronization from MLPlugin
Fix IntelliJ warnings in AbstractSvmModel