Support spherical honeycombs

[roice3]

We need a set of methods like the current hyperboloid methods, but for S^3.

My main question: Do we want to try to mash this all into one shader, or do we want a separate shader for the spherical (and maybe euclidean) cases? A separate shader would avoid some branching within the shader execution, but that might be trivial performance-wise. I'm leaning towards two separate shaders, one for spherical+euclidean, and a second for hyperbolic. We could, however, share some of the include shader files between the two.

[henryseg]

Let's not stop at hyperbolic, spherical and euclidean - there are the other five Thurston geometries to do as well.

[henryseg]

I think euclidean would be a good next case - it will make clear how things should be abstracted so as to leave as few possible differences between the geometries as possible.