Modifying fields to plot for 2D slices

[jorafb]

Hi Evan,

It would be nice to have the option of "modifying" the field to be plotted in slices. For example, if someone wants an equatorial slice of the angular velocity (v_phi/r) but only saved v_phi, an option for taking the relevant ratio would be useful.

[evanhanders]

This would definitely be great to implement! Plotpal currently has a structure like this for 1D profiles (you can feed it a lambda function that does some post-processing before plotting), but not for 2D slice data. I think we should be able to do something similar.