Allow for different mesh size in x and y in tidy3d

[mdecea]

Right now, it seems like the tidy3d simulations in modes.py only allow for uniform grid with the same number of cells in x and y., which is pretty inflexible.

Important note: although in the argument description in the docstring it says that resolution is the number of pixels/um, that's not true! Resolution is indeed the total number of cells in the x and y directions.

I want to request for at least the possibility of allowing different number of cells in the x and y directions. I have it implemented in my local copy of gdsfactory, so if you agree I can just create a PR. Just want to ask first because it is likely that some code will have to change as I have it implemented taking as parameters resolution_x and resolution_y instead of resolution (for example get_simulation_xsection.py will for sure have to change.

[simbilod]

You could also make resolution_x and resolution_y new Optional parameters that override resolution if set