fit() and predict() methods

[sgbaird]

Not necessarily critical, but struck me that it might be good to have a mdl.fit() and a mdl.predict() relative to what happens in train_crabnet.py to make it easier to "train once" and then reuse the model several times and for consistency with similar packages. Then I realized there is a model.fit() call inside of train_crabnet.py relative to model = Model(CrabNet(...), ...). If I want to "train once" and then reuse the model, how would you suggest going about that?