Export DG function correctly to paraview

[Stein]

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DG fields in Paraview

Things wrt IO never seem to stabilize, there is always the N+1 formats. Here is a rundown as of Feb 2026: XDMF: currently the preferred format for reading in meshes in parallel, as it uses hdf5 to store heavy data. However, the format has not been maintained since ~2021, and supports limited visualization of higher order fields/DG fields. VTX (.bp): Scalable output format similar to xdmf, but uses adios2 for the IO. Not good for reading in meshes, ref: VTK support for unstructured grids with g…