Break up the volume renderer code

This splits the `VolumeRenderer` class into several different classes. It should make it easier to add new `Actor` objects to the scene.
Could you add an `actor` property to `VolumeRenderer` and pass that in as `volume_actor` to these `add_actors_to_scene` methods. That way, `VolumeViewer` doesn't have to dig too deep into `VolumeRenderer`.