Add support for pre-initialized classifier models

[flekschas]

Currently, we only allow the user to choose between SKLearn classifiers and enforce the initialization of a new classifier. (See https://github.com/Novartis/peax/blob/develop/server/classifier.py#L104)

It should also be possible to start with an already initialized classifier. Technically this could be achieved by passing in a custom constructor that returns an already initialized classifier. In this case, Peax should only assume that the classifier's API matches SKLearn's API and crash mindfully if it does not. I.e., it should print an info message that it's the user's responsibility to ensure that the API conforms to SKLearn.