VTK files contains UInt32

[Sigvald]

Trying to read the files generated by FEniCS 2019.1 (and I think, also the dev version) in the Paraview 5.8.0 (from February), results in the following error: “Error reading cell offsets: Unsupported array type: vtkUnsignedIntArray”. Indeed, inspecting the file cell offsets is of type UInt32, which worked in Paraview 5.7, but which is apparently not according to the VTK specification. See also: https://discourse.paraview.org/t/cannot-open-vtu-files-with-paraview-5-8/3759/2 . The files in dolfin/io named VTKFile.cpp and VTKWriter.cpp seems relevant. (A quick workaround is to change the type in the VTK-files to Int32).

Edit: I see now that as of Paraview 5.8.1 unsigned will be accepted again. Perhaps not necessary to change it then.