Testing HoudiniScene with flattened geometry.

Adding tests showing that it doesn't support flattened geometry (all in one SOP) unless every level of the hierarchy has its own shape.
Testing SceneCache ROP for flattened geometry.

Adding test showing that it doesn't work for flattened geometry, because the HoudiniScene is returning the wrong results.
Fixed HoudiniScene bugs for flattened geometry.

Gaps in the hierarchy are now supported, and this commit fixes the tests from the previous two commits.