Changed CustomSourceTime to automatically modulate at freq0

[caseyflex]

Changed CustomSourceTime to accept a (real or complex) envelope, and automatically multiply it by e^{i \omega_0 t}. This particular complex modulation is needed for it to accept the phase shift from the different source types (e.g. e^{i k_0 \cdot r} for PlaneWave). Currently, one could put in cos(\omega_0 t) time dependence (or even something stranger), which leads to inaccuracies when attempting the phase shift. Having automatic complex modulation removes these inaccuracies.

[tylerflex]

Looks fine to me, just a couple comments about clarifying in the docstrings what the source amplitude looks like.

[momchil-flex]

I think this makes a lot of sense now, the extra parameters like freq0 and offset are no longer decoupled from the actual signal. Only fwidth still kind of is right? But I guess that's fine.

[caseyflex]

I think this makes a lot of sense now, the extra parameters like freq0 and offset are no longer decoupled from the actual signal. Only fwidth still kind of is right? But I guess that's fine.

yeah, that was my thinking. fwidth is still just an estimate of the bandwidth, which should be fine

[tylerflex]

feel free to rebase and merge when you are ready thanks!