Use VTK for comparing results

[BenjaminRodenberg]

Currently we use this script to compare the results. There are two problems:

• Bash is hard to understand and debug (at least for me)
• It does not compare the actual data, but the content of the files. If the format changes, but the data is still the same this is no regression, since we want to test the data and not the format. However, the tests will fail, since the files differ (e.g. https://github.com/precice/systemtests/pull/273).

I would suggest to use VTK for comparing results. VTK is available in C++ and python. I have some limited experience with vtk in python and think this should be doable with a small python script using VTK. Noteworthy: VTK Is a more or less heavy dependency.

[BenjaminRodenberg]

https://github.com/precice/systemtests/pull/273#pullrequestreview-572184832 reminds me of one case we cannot compare with vtk: comparing iteration numbers is actually quite nice for some cases.