fit() and predict() methods

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?