Support sphere unions as shapes (GSD)

We're defining unions of spheres as possible shapes in .gsd files. It would be great if OVITO could render them. I am creating this issue now, i.e. before the corresponding PRs are merged, so that OVITO developers are in the loop.

Note: GSD doesn't specify colors. An initial implementation could render the sphere unions with uniform color, and later on make it possible to read in external definitions with individual colors for every bead.