Reuse mesh when saving to VTX

[massimiliano-leoni]

Similarly to Fides, VTX too supports saving the mesh only once and reusing it for all time steps. The reason this didn't work out-of-the-box when we implemented the corresponding functionality for Fides was that, while the mesh was only saved once and the actual finite element functions were saved many times -- which is correct -- the functions "vtkOriginalPointIds" and "vtkGhostType" were saved only once too as they are saved when saving a mesh. For some reason, ADIOS2 needs them to be saved for each time step, probably because it needs all functions to be saved at all time steps. See https://gitlab.kitware.com/vtk/vtk/-/issues/19033 for more details.

This pull request follows in the footsteps of the one that implemented mesh reuse for Fides but also fixes this issue by caching the data for "vtkOriginalPointIds" and "vtkGhostType" when the mesh is written and saving additional copies of these functions at each time step.

The main change is in VTXWriter::write where the code now checks if the policy is to always write the mesh [or if it's the first write], in which case it writes the mesh before writing the functions. In case the policy is to reuse the mesh, it only writes the necessary data [as discussed above] and the functions.

[jorgensd]

A few things that I think could be useful to consider:

1. Python interface (we want it to be possible to control it from there)
2. Single constructor. The engine (BP4) should be possible to control
3. Does VTX support having meshes on a sub-set of time steps? i.e. say we have 10 time steps, but the mesh is constant from step 0-3, step 4-8 and step 9. If it is possible to then just store 3 meshes, the policy should be input to the write function rather than the constructor.
4. Does VTX support the mesh topology changing? If not, can we just write the geometry and not topology to file for every rewrite?

[jorgensd]

The only thing I'm missing from this PR is a test in C++ or Python on usage, as adding it with no testing in the library is a way of introducing bugs.

[massimiliano-leoni]

@jorgensd I added a unit test.

[jhale]

Some small comments then I think this can be merged.

[jorgensd]

Added a test that checks if data is placed in appropriate steps with the VTXMeshPolicy.update and VTXMeshPolicy.reuse.
I guess we should add a note somewhere that reuse can only be opened in Paraview>=5.12

[massimiliano-leoni]

Anything else that I can do for you guys? :)