VTXWriter check unique names

[schnellerhase]

Following MWE previously failed on read in Paraview not on output, due to duplicate function name f.

from pathlib import Path

import dolfinx
from mpi4py import MPI

comm = MPI.COMM_WORLD

mesh = dolfinx.mesh.create_unit_square(comm, 10, 10)

V = dolfinx.fem.functionspace(mesh, ("P", 1))
f = dolfinx.fem.Function(V, name="f")

W = dolfinx.fem.functionspace(mesh, ("P", 1, (3, )))
g = dolfinx.fem.Function(W, name="f")

with dolfinx.io.VTXWriter(comm, f"{Path(__file__).stem}_output_test.bp", [f, g]) as writer:
    writer.write(0.0)

ADIOS2 requires unique names - https://adios2.readthedocs.io/en/latest/api_full/api_full.html#_CPPv4I0EN6adios22IO16InquireAttributeE9AttributeI1TERKNSt6stringERKNSt6stringEKNSt6stringE.
This adds a check to the name extraction. Also removes raw pointer logic.