Introduction of training phases

[GilesStrong]

Idea

Currently, training of a model uses the same settings, callbacks, and data for the entire training process. It could well be the case that the user wishes to change certain aspects at set points during the training. A simple example could be changing the LR cycle callbacks. A more complicated example could be changing the training data during training, e.g. from Delphes to Gent4 simulations. Another example could be starting with parts of the model frozen, and then unfreezing them at a set point (e.g. pre-training a part of the model to work better on low-level information before introducing high-level information)

This could potentially be allowed by defining training phases, each with their own sets of settings, callbacks, and data.

This idea will no doubt require large changes to fold_train_ensemble, and some sort of 'trigger' callback to move to the next training phase.