Just implemented this and cleaned up some other pieces of ChromaticConvolution.draw().

I think the reason multiplying the Bandpass by the separable SEDs feels inelegant to me is just that it feels like the SED should be multiplying the object, not the Bandpass.  (Throughputs > 1.0 feel wrong).

It occurs to me that multiplying an object by an SED could actually make sense, especially if the object is a GSObject.  This could be the recommended way to create a `Chromatic`, for instance.  I'm not sure what the use case (other than `ChromaticConvolution.draw`!) would be for multiplying a chromatic object by an SED, but that could probably be implemented too, similarly to how ChromaticAffineTransform implements shears/shifts/etc.