Replace the use of the RelevancyIndex with a Classifier instance

[Purg]

It seems like the RelevancyIndex interface and intent is just a specific use of a SupervisedClassifier instance upon a set of DescriptorElements. The IqrSession structure is the only thing currently (within SMQTK at least) that calls the RelevancyIndex.rank method (aside from tests), thus the only thing that requires the specific positive/negative classifier training and application to a DescriptorElement set. A more generalized structure here would be for the IqrSession to take in a SupervisedClassifier implementation config (instead of a RelevancyIndex) and IqrSession.refine to define the training and application logic that is specific to the IQR process instead of fragmenting it out into a separate class (the RelevancyIndex).

This would also work to address opening up the use of other supervised classifiers for use in IQR refinement instead of currently needing to implement an analogous RelevancyIndex.