Added public access to the underlying SceneInterface
Added Push Parms button and pushToHierarchy virtual function.

This allows a user to update the GeometryType and filter values throughout the hierarchy without having to collapse and re-expand. In the future, this may be changed to push hierarchy changes as well as parameter values.
Merge pull request #89 from andrewkaufman/pushToHierarchy

Houdini SceneCache OBJs can push parameter changes