Add support for higher order polynomials to VTKWriter
New versions of paraview support visualization of arbitrary order polynomials. It would be very nice to have support for writing the corresponding files in dune. While paraview does the visualization using internal subsampling, there are still many benefits to this approach compared to our subsampled writer:
- File sizes are reduced.
- Files still contain all the information.
- More flexibility in visualization: One can e.g. visualize curved element edges which is not possible if subsampling happened externally.