Fix FileWriter for .pvtu files

Used https://kitware.github.io/vtk-examples/site/VTKFileFormats/ as guide

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• 92d5bdf4 - Fix typo in CONTRIBUTING.md

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• 92d5bdf4...7d9867af - 8 commits from branch irmb:develop
• 91018628 - Merge branch 'develop' of https://git.rz.tu-bs.de/m.schoenherr/VirtualFluids_dev into develop

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mentioned in commit 99ba94fc