disable automatic processing of tet mesh results

[shimwell]

The default behaviour when running a tet mesh is to process the results and produce a vtk file.

However this can be disabled with UnstructuredMesh.output = False

Then we can call the write_data_to_vtk function in the wrapper https://github.com/openmc-dev/openmc/blob/eacd8f6bba414cac12f65059a3c17226fd0a201f/openmc/mesh.py#L722

We could do with adding a scaling factor to the write_data_to_vtk so that the results can be written out in whatever units required (e.g Watts would need scaling from eV to joules and by fusion power)

[RemDelaporteMathurin]

@shimwell can we close this?