improve mode_solver

[joamatab]

At the moment the mode solver RectangularDielectric in tidy3d is somewhat to strip/rib

How can we use tidy3d mode solver to ?

• solve waveguides with varying refractive index
• solve multi-layer waveguides
• compute mode leakage to substrate
• plot lines corresponding to the interfaces between layers and sidewalls, so the mode profile is overlaid with the layer stack and geometry

[momchil-flex]

Related to #1628

Generally all of those are possible now but not with the RectangularDielectric plugin. Not sure how much bandwidth Lucas will have but he's promised to implement some of these in the plugin.

[lucas-flexcompute]

The only request not covered by the PRs is mode leakage calculation, because that seems to be problem-dependent. I don't think we can provide a general and robust solution covering all cases for layered media…