Cannot write function to XDMF with DG elements

[dokken]

@RemDelaporteMathurin

V = fem.FunctionSpace(msh, ("DG", 1))

u = fem.Function(V)

xdmf_file = XDMFFile(MPI.COMM_WORLD, "u.xdmf", "w")
xdmf_file.write_mesh(msh)
xdmf_file.write_function(u)

This is an explicit change to highlight the limitations of the XDMF file format, as in comparison to VTX.

In DOLFINx<0.7.0 any function was interpolated into the function space of the mesh geometry (N-th order Lagrange space, N usually 1).
Now you have to interpolate this function into (“CG”, 1) explicitly to save it to xdmf, or use the VTXWriter, which supports arbitrary order “CG”/“DG” elements.