Fixed animated ROP test.

It wasn't actually verifing the results on any time but 0... luckily it was actually working.
Fixed bug with writing re-rooted scenes.

They would throw if caching multiple frames (as shown in the test). Now they correctly check for an existing re-root child before creating a new one.
Merge pull request #91 from andrewkaufman/reRootingPatch

Houdini SceneCache ROP Re-rooting patch