Simplified base model API

[tennlee]

The current BaseForecastModel base class is more complex that is required for simple model types, resulting in increased complexity for some model developers. An interface more akin to training.wrapper.wrapper.predict.predict.Predictor would provide something simpler to get started with, and could be set up as a parent class of BaseForecastModel. It could be called something like BasePredictor or similar.

The BaseForecastModel class is currently defined in zoo.model.BaseForecastModel. In an ideal world, there would be something like pet.core which defines key abstract base classes for re-use across the codebase (e.g. in both zoo and training).