Feature/263 refactor fedbiosklearn to hierarchy classes

This merge request contains a new implementation of the sklearn models based on a hierarchical design. The base class SKLearnTrainingPlan should never be instantiated on its own, but should be inherited from by other concrete classes that wrap specific sklearn models (e.g. FedSGDClassifier).

I have fully removed all the traces of the old implementation, hence this implementation will not be backwards-compatible. This change can be easily reverted if needed.

Also, as decided, I have removed the BernoulliNB and GaussianNB models.