>Any reason not to put the default integrator in the function definition (i.e., define the kwarg using integrator=galsim.integ.midpoint_int_image)?

I didn't have any particular reason for this before, but now I've retooled `ChromaticObject.draw` so that the default integrator depends on whether 'bandpass' has the attribute 'wave_list' or not.  So now I think integrator=None by default makes sense. :)