refactor to load recommender models generically from Classifier.load()

Classifier.load() using a saved SVM model does return an SVM classifier, but it doesn't work due to missing initialization of libsvm. It only works if you use SVM.load()

This change moves the classifier specific initialization into an on_load() instance method so that Classifier.load() can be used for all types of classifiers.

It seems to work, but is not well tested.