Properly deal with recent VTK
Since the interface changed significantly in VTK 9, certain feature gaurds are necessary. The new API can be detected with the definition of VTK_CELL_ARRAY_V2
(see https://vtk.org/doc/nightly/html/classvtkCellArray.html, C-f GetPointer).
The second problem are the renamed CMake packages, they all remvoed the vtk
prefix. We may get away with just using the prefixed ones, since they only throw an error.
Third: The first commit by Rene seems to fail most CI tests... this needs investigating, since at least some complain about missing packages, which is odd.