Definition of plane_center for bend mode solving

[mpasson]

It seems that for modal simulation tidy3d requires the definition of the center of the simulation.

I added a sensible default to the inner _compute_mode call, but right now this argument is not exposed. Do you think it would be better to expose it, maybe as an additional attribute to the mesh object?

[flaport]

Yeah, exposing it makes sense. Do you want to make a PR?

[mpasson]

Hi! Thanks for the feedback! I added it to the mesh object, and made a jupyter example with a bent waveguide on this pull request. Let me know if it is ok or you prefer something different.

[flaport]

Thank you!