GetCell() no longer thread safe for Lagrange and Bezier cells
vtkDataSet::SetCellOrderAndRationalWeights(vtkIdType cellId, vtkGenericCell* cell) is not thread safe due to the invocation of SetActiveAttribute("HigherOrderDegrees", vtkDataSetAttributes::AttributeTypes::HIGHERORDERDEGREES) != -1) every time vtkDataSet::GetCell(vtkIdType cellId, vtkGenericCell* cell) is called.
This results in insidious threading failures.
Initially involved were TJ and Florian Maurin
Edited by Mathieu Westphal (Kitware)