Train diagnostic figure update

[jeffjennings]

NOTE: This PR should only be reviewed after #207 and #206 are merged into main and main merged into this branch (it was branched from 207, which was branched from 206)

• Adds some new plots to the diagnostic figure that's generated at some chosen interval during a training loop:
  • difference image between model image at current iteration and that at the previous instance of the interval
  • flux of model image vs iteration
  • learning rate vs iteration
• Tracks model flux and learning rate in training loop
• Some general cleanup in the training and cross-val classes

[jeffjennings]

Example of the new train_diagnostics_fig (convergence was reached at shown epoch)

[iancze]

Excellent! Would also be good to roll into a tutorial or other narrative documentation.

[jeffjennings]

Thanks! I've made a note about adding this in #63; there might be a good earlier place in the docs to put it, but #63 is looking like the hub for the next tutorial additions to the docs.