Some updates

[jcapriot]

This cleans out all of the notebooks' cells, as well as updated a few non-running notebooks.

There's also a lot of cleanup in the dc simulations to take advantage of the improved fields object, and make use of caching the simulations when possible to make viewing plots a little more responsive.

[lheagy]

It looks like there is one other file that got added in the mag directory along the way that I think could be removed?

[jcapriot]

Thanks Lindsey, done and done

[lheagy]

Many thanks @jcapriot!