Hiding the top level SceneCache OBJ during expansion.

Also using setDisplay instead of setVisible, to control visibility of a single node without effecting parent nodes. On a heavy asset, this reduces expansion time from 442.3s (7.38m) to 17.12s.
Moved the work of sceneChanged() into updateState()

By deferring calculations until the node actually cooks, we shave another 12% (2 seconds) off the expansion time.
Merge pull request #129 from andrewkaufman/expansionTime

Houdini SceneCache Expansion optimizations