Add top and bottom cladding geometry in Meep/MPB mode solver

[jonbenfri]

Is your feature request related to a problem? Please describe.
I want to model designs with only a bottom cladding (no top cladding). Currently, the mpb mode solver code assumes full cladding.

Describe the solution you'd like
Use two geometry components for cladding: one for the top cladding and one for the bottom cladding.

More specifically:
Copy the geometry format from the Tidy3D mode solver.

Extend the code for get_mode_solver_rib.py, get_mode_solver_coupler.py, and get_mode_solver_cross_section.py.

In the Tidy3D mode solver code, there are additional kwargs:
box_material: bottom cladding material
box_thickness: thickness of the bottom cladding

I will try to work on this and submit a pull request.

[joamatab]

Awesome, looking forward to your PR