Single Broadband Adjoint Source

[tylerflex]

When the objective function depends on:

• the data from a single monitor
• only a single non-frequency coordinate (eg only one mode_index or direction)

We can implement the adjoint source as a single broadband source (instead of a source for every frequency) and then normalize the adjoint fields using the VJP amplitudes.

This would eliminate the need for very long run times in common, simpler broadband adjoint problems, especially involving mode monitors. It would make it more convenient and feasible to do finely discretized broadband adjoint problems in tidy3d.