Refactor model class hierarchy

[crasanders]

The current model class hierarchy is a bit confusing:

• We have the AEPsychMixin, which should be made into a proper base class because it's being used as one instead of as a Mixin. This would make the code more straightforward and allow us to get rid of the clunky ModelProtocol.
• A lot of model classes inherit from GPClassificationModel, even if they don't actually do classification (e.g., the Beta and Ordinal outcome models). Instead we should have a VariationalModel superclass that all these different models inherit from.

See the class structure in the ax-backend branch for an example of what the class hierarchy could look like.