Implement a new method that predicts probabilities, where it makes sense

[VolodymyrOrlov]

Classification algorithms usually offer a way to quantify certainty of a prediction. In Scikit learn a method that returns probability estimates for all classes is called predict_proba.

We need a similar method in SmartCore. One way to do it is to define a new trait Classifier, that will have a function predict_proba, and implement this trait for every algorithm where predicts probabilities makes sense.