VTXWriter: support file modes

[schnellerhase]

Adds support to ADIOS2Writer and VTXWriter to open files in append mode (following interface of VTKFile). Enabling usage such as

import dolfinx
from mpi4py import MPI

mesh = dolfinx.mesh.create_unit_square(comm := MPI.COMM_WORLD, n := 10, n)
V = dolfinx.fem.functionspace(mesh, ("CG", 1))
f = dolfinx.fem.Function(V)

# this usage previously only held the 9th iterate in the output, i.e. was not supported
for i in range(10):
    f.x.array[:] = i
    with dolfinx.io.VTXWriter(
        comm,
        "out_a.bp",
        "a",
        [f],
    ) as writer:
        writer.write(i)

# now holds equivalent output to:
with dolfinx.io.VTXWriter(
    comm,
    "out_w.bp",
    "w",
    [f],
) as writer:
    for i in range(10):
        f.x.array[:] = i
        writer.write(i)

Demonstrates the new feature, with simplified I/o logic in the naiver stokes demo.

[jhale]

Remove "one of", automatic when using "or".

[jhale]

else if and else?

[jhale]

Did you consider just wrapping adios2::Mode directly as an enum?

[jhale]

Use non-owning std::string_view

[jhale]

Leaving aside Enum vs no Enum, it's generally most transparent to map modes directly e.g. "Read" -> "adios2::Mode::Read". Then I have a clear idea what "Read" means in the context of the underlying library.

[jorgensd]

I'm a bit sceptical about adding an "append" mode, as the VTXScheme shouldn't (or at least couldn't previously) be modified at runtime with ADIOS2, which is why the initializer takes in the functions/meshes to store at each time-step, instead of following the XDMF file version where we write when we can.

adding an append-mode needs us to revise the internal logic to ensure for consistent input at multiple steps.
We would also have to thoroughly check which variables ADIOS2 now allows to be time-dependent (last section of https://adios2.readthedocs.io/en/latest/ecosystem/visualization.html)