Warn when mode solver pml covers a significant portion of the mode plane

[caseyflex]

Partially addresses this issue:

#2277

Adds a warning when the PML covers a significant portion of the mode plane, and makes it easy to get the mode PML thickness (e.g. for GUI plotting).

Doesn't change the mode PML behavior, API, or plotting.

[caseyflex]

@momchil-flex fyi. You mentioned changing the plotting for mode monitors and mode sources to show the mode pml. I didn't include this here because I wasn't sure about it -- it would clutter the plotting, and maybe it's not that critical most of the time? But we could consider including that or unifying the plotting between the ModeSolver and the mode objects in FDTD simulations (currently ModeSolver shows the pml while FDTD mode objects do not). It would be easy to include in this PR if we decide to make that plotting change.

[yaugenst-flex]

Thanks @caseyflex looks good to me! We really need a config though for these globals... 😄

[caseyflex]

@e-g-melo can we merge this?

[e-g-melo]

Hi @caseyflex! Thanks for this work!

I am ok with the changes to plotting.

After trying it, I suggest reducing that threshold to 50% of the mode plane.

[momchil-flex]

Sorry, one thing this is making me think about is that e.g. in RF CPW simulations, the mesh could be much much finer around the waveguide than at the outer edges of the mode plane. So while the physical area of the PML may appear large, there is no problem because the majority of the grid cells are outside of PML. I feel like generally checking on the grid points level rather than physical area might be better?

[caseyflex]

@momchil-flex I reworked it so the warning is now based on num_cells (although the area functions are still available if needed)

[momchil-flex]

Looks good!