Break scene.transforms into subpackage

Just reorganization. All scene/visual examples work as previously.
Ready for review.
Maybe we should change this to `BaseTransform` to make it clear it should be subclassed, and change the filename to match. When I originally saw the imports in `__init__.py` with `from .transforms import Transform` I was confused. Calling the file and class `BaseTransform` wouldn't have confused me.