Allowing .lscc files to be loaded
Fixing SceneCacheNode function signatures

Making scene() static since it doesn't use any member data or node parameters. Making get*() const.
renaiming "build" to "expand" and "clean" to "collapse" to match IECoreMaya naming
Separated Expand and Collapse functionality.

SceneCache OBJs now expand and collapse from separate functions (and buttons). The Expanded toggle marks the current state, and may be queried by external processes (such as a ROP) to determine if the user has fiddled with the underlying scene.
Changed behaviour of SubNetwork expansion to supress child objects.

This is necessary to ensure that objects are not expanded prior to children of the same level, providing a coherent definition for the expanded/collapsed state of each node.
Allowing custom attribute handlers to be registered with HoudiniScene
OBJ_SceneCacheTransform registers the LinkedScene attribute with HoudiniScene to represent unexpanded transforms as links.
LinkedScenes can now be written with the SceneCache ROP.

The ROP will now write attributes, the primary use of which is the link attribute for lscc files. Required wrapping the live HoudiniScene with a LinkedScene when writing non-linked files, to ensure full expansion of links.
Merge pull request #12 from andrewkaufman/houdiniLinkedScenes

LinkedScenes in Houdini