plots for brain

it’d also be nice to have some way of knowing for the reconstructed brain objects (with nearest neighbor matching) which electrodes were matched with which locations in the model, and how many were used.

e.g. you could attach two additional locations matrices to the Brain object returned by predict that get used by `plot_locs()` (one for the original brain locations and one for the model locations)

Then plot_data could plot:
- black electrodes: locations in original dataset
- red electrodes: nearest neighbor matches to the original locations (connect with lines?)
- blue electrodes: locations in *model*