Feature request: Enable the user to upload their own 3D coronal magnetic field data

[dmueller3000]

Feature request:
Enable the user to upload their own 3D coronal magnetic field data in FITS format (or else) instead of using the precomputed PFSS fields.

This will require a description of the input format and UI changes to load the data.

Justification (provided by S. Bale):
We’re finding a lot of variability when comparing to PSP data. Source surface heights are variable, input data (GONG vs HMI, ADAPT or not, etc). So probably having the capability to load a model in FITS would be more powerful ultimately.