What does this MR do?

Implement a SubsamplingVTKWriter for full precision VTK output (in higher polynomial orders).

• Add SubsamplingVTKWriter to OutputWriter.
• Add config file key output.subsamplingLevel
• Use default value of 0.

Is there something that needs to be double checked?

This is a follow-up to !5 (closed) where higher residuals where noticed when writing a subsampled output. Deemed artifacts, these features might indeed be part of the actual solution.

• Is it okay that no test uses subsampling?

  Output still shows characteristic oscillations of residuals for subsampling levels > 0. Benchmarks indicate that this is a true feature of the solution.

Can this MR be accepted?

• [ ] Added test to ...
• Pipeline passing
• Added entry to CHANGELOG.md
• Add deprecation warning to CHANGELOG.md

Related issues

Closes #11 (closed)