Implemented light add/remove/show/hide in IPR renderer

This addresses #874. There are unit tests for both actually adding and removing lights from the hierarchy and also keeping a fixed hierarchy but turning visibility attributes on and off.

As best as I can tell, there's not actually a way of deleting a light in 3delight, so I just turn them off with RiIlluminate when they've been hidden/deleted, keeping them around to be resurrected if they get shown/recreated.