VTKHDF proposal: Support splitting datasets in multiple files

[Jacques-Bernard (Lekien)]

@Louis_Gombert

The VTKHDF reader currently does not use HDF/MPI-IO, but the VTK MPI implementation, which makes sense as a VTK filter and is easier to integrate with the rest of the VTK API.

I completely understand, functionality above all.
It could be interesting in an HPC context to have a reader using HDF/MPI-IO (this nevertheless remains to be verified).