BUG: use addArray instead of setScalars to avoid overwriting active array

Using AddArray instead of SetScalars avoids overwriting of the active scalars, which seems to be the source of the problem described here: https://discourse.vtk.org/t/point-data-from-vtkprobefilter-lost-after-vtkselectenclosedpoints/130 It is not clear to me from the description in the docs, what the actual difference is between the two methods in this case.