Clarify learning interfaces

* Make threshold mandatory
The output of ``predict`` for an example was either the score or the
class. This is a side-effect of ``clearThreshold``. To clarify that
unexpected behaviour three different types of predict (predictScore,
predictClass, predictProbabilty) were introduced; the threshold is not
longer optional.

* Clarify classification interfaces
Introduce BinaryClassificationModel for all models that derives a binary
classification from a single weight vector. Comprises the tresholding
functionality to derive a prediction from a score. It basically captures
SVMModel and LogisticRegressionModel. The additional trait
ProbabilisticClassificationModel defines probabilities.
ProbabilisticClassificationModel and ClassificationModel are now also
applicable to multi-class problems.