output connectivity in `hdf5` files - add `vtkHDF` support ?

[tbeltzun]

What is the requested feature?

.hdf5 fields are currently written as a 1d stream of values.

E.g. for an acoustic solver the nodal pressure field is three-dimensional, we thus loose the connectivity in the process. It is trivial to reconstruct a Q1 mesh with a single solver, but visualizing a solution on arbitrary unstructured mesh points requires faces connectivity.

As discussed on slack, adding a VTKHDF extension to fileIO/vtk is probably the best approach.

Describe the solution you'd like

Having the output field connectivity stored somewhere would allow mesh reconstruction from post-treated data. This could be an optional flag.

Related: https://github.com/GEOS-DEV/GEOS/issues/2592.