Mark the entire Accelerators Piston module as legacy as of 6.3

Accelerators/Piston/CMakeLists.txt

Accelerators/Piston/module.cmake

Accelerators/Piston/vtkDataSetToPiston.cxx

Accelerators/Piston/vtkPistonAlgorithm.cxx

Accelerators/Piston/vtkPistonContour.cxx

Accelerators/Piston/vtkPistonDataObject.cxx

Accelerators/Piston/vtkPistonMapper.cxx

Accelerators/Piston/vtkPistonReference.h

Accelerators/Piston/vtkPistonScalarsColors.cxx

Accelerators/Piston/vtkPistonSlice.cxx

Accelerators/Piston/vtkPistonSort.h

Accelerators/Piston/vtkPistonThreshold.cxx

Accelerators/Piston/vtkPistonToDataSet.cxx