Adding HoudiniScene::node().
Fixing constness of scene() method.

Also preventing bad calls if the necessary parameters don't exist. This avoids ugly error messages when dealing with broken nodes.
Adding ForceObjects parameter to the ROP.

This allows users to force specific objects to expand (and force the unlisted objects to be links if possible). An empty value will behave as normal (no forced behaviour).
Merge pull request #97 from andrewkaufman/ropLinkControl

ForceObjects parameter for SceneCache ROP