code: def extract_geometry(self):
          
          gf = vtk.vtkCompositeDataGeometryFilter()
          gf.SetInputData(self)
          gf.Update()
          return wrap(gf.GetOutputDataObject(0))
docs: Combines the geomertry of all blocks into a single ``PolyData``
          object. Place this filter at the end of a pipeline before a polydata
          consumer such as a polydata mapper to extract geometry from all blocks
          and append them to one polydata object.