Tag Improvements in Houdini

Specifically:

 - tagFilter now prevents unnecessary expansion in SceneCache Xform
 - tagFilter exists on the SOP and is pushed through during Xform expansion
 - shapeFilter exists on the Xform and is pushed as well
 - Users can add new tags to OBJs via the ieTags parameter
  - ieTags must be a space separated list of strings
  - Scene Cache OBJs have this by default, but any OBJ can define tags by adding an ieTags spare parm
 - Users can add new tags to SOPs via PrimGroups
  - PrimGroups prefixed with "ieTag_" will be extracted as tags. E.g. a PrimGroup named "ieTag_metal_rust" will tag all included shapes with "metal:rust"
  - Note that the new tags will be available in the tagFilter on import, but will not be converted to PrimGroups. The groups are purely to mark something for export.