Update spacecraft charging test

[RemiLehe]

The spacecraft charging test was not properly correctly the electric field in the simulation with callback functions.

The purpose of the callback functions here is to correct the electric field, so that the charge on the spacecraft computed using Gauss law (from the electric field) matches the actual charge (calculated from collecting particles on the embedded boundary).

However, the current script corrects a copy of the electric field. The actual electric field in the simulation was not corrected. This can be seen from the print statements when the code is running. (The print statements print the charge from Gauss law and the actual charge, and they did not match with the current version of the script.)

[dpgrote]

This could be cleaned up a bit more. Since self.normalized_Er is an already a numpy array, the [...] is not needed. And the same for Ez and phi below.

[dpgrote]

This looks good.

I would comment for the record that using MFIter type loops would be better here (since it would avoid global copies and would run on GPU), if there is ever time to update this example.

[RemiLehe]

Yes, agreed.
I think I could do a follow-up PR in the next week or so, using pyamrex to do the above.