Fix Sup3rGan interface

[malihass]

Adding additional abstract class that contains training functions that can be used by other types of models such as conditional moment models.
Remove Sup3rGen which was only useful in the context of conditional moment learning. This class will be replaced by Sup3rCondMom when conditional moments models are ready to be uploaded.

[grantbuster]

@malihass I like this a lot better, what do you think? Looks like it turned out to be mostly renaming/organizing but i think it's a more logical organization.

One comment: Would you want to remove meta, training_features, and output_features from AbstractSingleModel? These are in AbstractInterface and I don't see a case where you'd use the SingleModel without the Interface. You could also do AbstractSingleModel as a subclass but i kind of like them separate, seems more modular.

[malihass]

Ah good catch, I forgot to remove them. I had to add them because I had other old methods that I had included in the SingleModel one before and eventually decided against it.
I prefer to have the two abstract classes separate for now in case we need these methods separately.