#581 Allow other transformations besides shift and expand to take functional arguments

The GalSim paper had the sentence "In addition, the shift, expand, dilate and flux rescaling transformations can take functions of wavelength for their arguments."

It seems (to me) a bit odd that we don't just allow everything to take such functional arguments.  The use cases might be less common for shear, rotate, and transform, but I thought the functionality should at least be available.  So this PR adds that functionality.  

The new sentence in the paper is now "In addition, the various transformations can take functions of wavelength for their arguments," in expectation of this change.
That was my initial reaction, but thinking of it as a handy way to simultaneously apply a shear and magnification (potentially for other reasons), it seems like there's not much reason to avoid allowing wavelength-dependent arguments.

However you might want to get another opinion since we're both kind of on the fence?