Scene Tags in Houdini

Tags can now be read/written by the SceneCacheNodes.

* The OBJ (transform) reader has a new tagFilter parameter
 * Uses Houdini string matching against available tags to determine visibility of child nodes during expansion
 * Available tags at each level are presented in the drop down menu
* HoudiniScene allows registration of custom tag readers, and the OBJ transform registers one
 * This simply translates the tags already existing in the scene file, and does not currently allow for live editing/addition of tags
 * However, tags are not maintained when using FlatGeometry mode
 * In the future, tag addition and FlatGeometry tags could potentially be made available using custom parameters and geometry prim vars
* The ROP will now write all incoming tags, unless at a link (in which case writting tags would throw an exception)