Allow local classifiers to be imported and registered

This merge request allows a user to import local classifiers without having to modify the iDQ codebase. This will be useful for developing and testing new classifiers, either from scikit-learn, Keras and pytorch or a homegrown classifier.

By default, it'll look in $HOME/.config/idq/classifiers.py for any classifiers that are registered, but this default path can be modified by setting the XDG_CONFIG_PATH variable, following the . Any classifiers that are registered can be called in the same way in a configuration file by setting flavor=<your_classifier_name> in the classifier section.