Fixed shared data problem

Fixed a problem where the global shared data was getting created and destroyed multiple times if there were multiple procedurals in a rib
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
Merge pull request #128 from davidsminor/autoInstancingGlobalHandlesFix

Fixed shared data problem
HoudiniScene can use a default time during evaluation.

Added get/setDefaultTime methods, which provide a default cooking time for methods that do not accept time as an argument (e.g. hasObject or childNames). In a HoudiniScene which points at a SOP, it is necessary to use time in these methods. The default time will pass through to children automatically. If left unset, CHgetEvalTime() will be used
for these queries.
Setting default HoudiniScene time from SceneCache ROP.

This prevents double cooking when the current cache time does not match Houdini's current eval time.
Merge pull request #127 from andrewkaufman/houdiniSceneDefaultTime

HoudiniScene Default Time