Fix bad merge with repeated code in GEO_PrimVDB
Fix bad merge in openvdb_houdini/Makefile
Merge changes from Houdini trunk. Notable changes:

- (Makefile) Fix bash error when HCUSTOM_EXPORT is empty
- (GEO_PrimVDB) Voxel evaluation now also handles bool grids
- (GEO_PrimVDB) Copy visualization options
- (GU_PrimVDB) Fix crash when saving NULL string attribute values to .vdb
- (Various) Save .vdb with blosc enabled unless HOUDINI13_VOLUME_COMPATIBILITY
  is set.
- (Various) Add missing "include what you use" headers
- (GeometryUtil) Add missing node interest in frustumTransformFromCamera()
- (ParmFactory and SOPs) Group parameters now have a selection button that
  allows one to use it for reselecting in the viewport.