Save metrics dictionary when saving model

[rwood-97]

Is your feature request related to a problem? Please describe. At the moment, we save:

• object checkpoint (i.e. ClassifierContainer object)
• model checkpoint (i.e. whole model)
• model state_dict check point (i.e. model params)

It would be good to save also the metrics dictionary so that we can look at how training progressed throughout each epoch. We could then reload and plot this without having to reload the entire object checkpoint.