[WIP] 3D Visuals

This pull request implements basic 3D visuals, like a 3D polyhedron or a 3D cube. I think it is a prerequisite for completing volume rendering as a visual. (https://github.com/vispy/vispy/pull/458)
+1 for combining into single `Cuboid` or `Cube` class, whichever is more common in 3D literature (I don't know)