Support list of DG-0 + other space in VTXWriter

[ordinary-slim]

Continuation of #3107 so that we're able to output both DG0 + another space in the same bp file. The previous PR by @jorgensd says:

I've decided to keep it such that only single function-space functions should be saved in a single bp file.
This could of course be changed now, to store a mixture of DG-0 and a single other element function.

This is up for discussion.

Since VTX is the recommended writer, I think it's necessary to support this feature. This code doesn't modify previous behavior regarding the written post files.

MWE

from dolfinx import fem, mesh, io
from mpi4py import MPI

cells_per_side = 4

domain  = mesh.create_unit_square(MPI.COMM_WORLD,
                                  cells_per_side,
                                  cells_per_side,
                                  )

cg1 = fem.functionspace(domain, ("Lagrange", 1),)
dg0 = fem.functionspace(domain, ("Discontinuous Lagrange", 0),)

u_cg1 = fem.Function(cg1, name="u_cg1")
u_dg0 = fem.Function(dg0, name="u_dg0")

expression =  lambda x : x[0]*x[1] 
u_cg1.interpolate(expression)
u_dg0.interpolate(expression)

writer = io.VTXWriter(domain.comm,
                      "result_folder/post.bp",
                      output=[u_cg1,u_dg0])
writer.write(0.0)
writer.close()

[jorgensd]

Currently DG-0 support in VTX is limited by: ornladios/ADIOS2#4179

[ordinary-slim]

Currently DG-0 support in VTX is limited by: ornladios/ADIOS2#4179

Thank you for the information, a bit of a bummer. This PR could still be useful for steady data.