Recognize pyiron HDF5 files as pyiron files

[jan-janssen]

Currently there in the HDF5 files there is no indication that they were written by pyiron. While this is not critical inside pyiron as we just assume that all HDF5 files in a given project are created by pyiron it would be very helpful for external tools like ovito to have some string to identify HDF5 file created by pyiron.