LinkedScenes in Houdini

LinkedScenes can now be read/written by the SceneCacheNodes.

* The OBJ (transform) readers have new expand/collapse buttons, and an associated expanded flag
 * This required changing the behaviour of SubNetwork expansion to supress child objects, which 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.
* HoudiniScene allows registration of custom attribute readers (using a similar mechanism to IECoreMaya), and the OBJ transform registers the reader for the link attribute.
* The ROP will now write all incoming attributes, though the only one currently provided by HoudiniScene is the link attribute for .lscc files. When writing non-linked files (.scc), HoudiniScene will ensure full expansion of the links.