Structure.from_permittivity_array

[tylerflex]

Fixes #1971

Open to changing the name Structure.from_custom_data since it technically just creates structure from custom permittivity data? maybe it could be generalized?

But basically I think this will simplify some adjoint demos so would be good to have in 2.7.4

[yaugenst-flex]

Oh nice this is really useful. I think the name is fine in principle. To be a bit more explicit, maybe something like .from_permittivity_array()?

[tylerflex]

from_permittivity_array

Sounds good.

BTW implemented in an example here
flexcompute/tidy3d-notebooks#173

[tylerflex]

@yaugenst-flex made name change here and in notebook

[momchil-flex]

Approved, ideally the procedure should be rebase and merge notebook PR -> update notebook submodule state here to develop -> rebase and merge this one.

[tylerflex]

rebase and merge notebook PR -> update notebook submodule state here to develop -> rebase and merge this one.

done, I'll merge when the tests pass again