Fix the vtk output for vectors in parallel
Description
This still used the old code path, resulting in a call to mesh_to_cube using a global mesh function which does not work anymore.
Add vtk output for a scalar and a vector quantity in a test and test some basic quantities in the header of the corresponding output files.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.