Houdini SceneCache Fixes

This addresses a few seg faults that could occur when using the SceneCache ROP in Houdini along with fixes for a few odd behaviours that came up while debugging the crashes.

 - Fixed crash when accessing a link that pointed to a non-existant file
 - Fixed crash when reading tags or links from regular Houdini nodes
 - Fixed time dependency issue when placing ieSceneCacheTransforms in subnets
 - Fixed broken test for LinkedScenes (required adding bindings for the new link attributes)
 - Fixed enable state of the expanded toggles