Sync from Houdini trunk

Some of the more notable changes:

GEO_PrimVDB.{cc,h}
    - Added GEO_PrimVDB::getSpaceTransform(const UT_BoundingBoxD &)
    - Fix GEO_PrimVDB::GridAccessor::updateGridTranslates() to properly handle
      case where the vertex has not been created yet.

GU_PrimVDB.{cc,h}
    - Add support for converting VDB to (optionally disjoint) sparse native
      volumes
    - GU_ConvertMarker API changes in the HDK

GeometryUtil.cc
    - Do not generate interior tetrahedron faces for VDB From Polygons

ParmFactory.cc, SOP_OpenVDB_Sample_Points.cc
    - Do not use the deprecated GEO_AttributeHandle class