Fixed angle simulations

[dbochkov-flexcompute]

Summary of changes:

• introduced field angular_spec in PlaneWave, which can take values FixedAngleSpec() and FixedInPlaneKSpec(). Those are currently empty specs, but in future we could add parameters to them.
• automatic time step reduction for FixedAngleSpec() due to a new stability criterion
• minor change in diffraction monitor data, so that results can be processed at multiple bloch vectors simultaneously
• btw, do you prefer FixedAngleSpec()/FixedInPlaneKSpec() or ConstAngleSpec()/ConstInPlaneKSpec()

example notebook https://github.com/flexcompute/tidy3d-notebooks/blob/daniil/fixed-angle/AngularFrequencyDependence.ipynb

[dmarek-flex]

Looks good to me, just a little clarification for beginners like me would be helpful!

[tylerflex]

Looks good to me. thanks @dbochkov-flexcompute ! Just a few minor comments but other than that, good to merge.