Parallel CGNS file i/o support

[mlohry]

This is a feature request to add support for parallel CGNS i/o equivalent to the existing support for HDF5 in relay i/o. CGNS itself is built upon HDF5, so conceptually one could use HDF5 directly to write files in the CGNS specification, or one could use the CGNS mid-level library for convenience.

In our CFD workflows, CGNS is widely used and supported in most common pre- and post- tools we use (visit+paraview+tecplot).

I can implement this myself and submit a PR if it's of interest.