broadband adjoint (2.5)

[tylerflex]

same as #1179 but rebased against 2.5, squashed, and polished up.

Note: works fine on version 2.4.2 but fails with version 2.5.0rc1 for some reason, looking into this.

Also note: there are two added tests (_test_adjoint_broadband ...) that should go in core.

Summary of changes:

• users can now add multiple frequencies to output_monitors and also different output_monitors can have different frequencies.
• internally, we maintain a sorted list of freqs_adjoint corresponding to these monitors, which become the freq0s of our adjoint sources.
• if not set by JaxSimulation.fwidth_adjoint, the fwidth of the adjoint sources is chosen to be smaller than some factor (0.1) times the minimum difference between the freqs_adjoint. This was chosen empirically to be small enough to have low error, but not be too small. When this is set, a warning is displayed to the user outlining that an fwidth was chosen automatically, which could lead to large run_time and some mitigation approaches for tightly packed frequencies.
• if not set by a new field JaxSimulation.run_time_adjoint, the run_time of the adjoint JaxSimulation, will be some factor (100) divided by the adjoint fwidth. Again chosen empirically to avoid early shutoff in the adjoint sims, while not being too long.
• The adjoint E fields are left unnormalized initially, but then are retroactively normalized by the adjoint source source_time spectrum at fwidths_adjoint. This is because we need to apply a different normalization frequency-by-frequency depending on the freq0 and fwidth of the adjoint sources, which may introduce amplitude or phase differences.
• All of the VJP calculations are vectorized over frequency. At the end of the calculations, we sum the results over the frequency axis (consistent with derivative rule: df(x,y)/dz = df/dx + dy/dy, forget the name of it.

Corresponding notebook changes: flexcompute-readthedocs/tidy3d-docs#296

I think that covers it, thanks in advance for the review!

[tylerflex]

ok I just added the source thing, fixed up changelog, squashed, etc. I think this should be ready to go assuming tests all pass.

Could I leave it to you to merge the core PR? maybe with this updated submodule commit?

[momchil-flex]

Backend tests pass, merging everything!