ENH: VTK6 migration, update vtkMRMLModelNode and sublcasses.
ENH: VTK6 migration, update vtkMRMLVolumeNode and its subclasses.
ENH: VTK6 migration, removal of update() from vtkDataObject.

Removal of Pipeline Update Methods from vtkDataObject.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_Update.