Draft: Koehler illumination

This branch includes the starting point for a simple Koehler illuminator - based on the rather hard-to-find "dissection microscope" (i.e. the v6 illumination dovetail) this allows several separate lenses to be used. It was used by the MPhys project on stereo illumination, and also by our early attempts to make a better illumination module. It probably won't be used in this form, but I'm making a MR to note what was going on and explain why the branch exists...