Add unit tests for HDF5Pump and HDF5Sink

I added some unit tests for reading and writing hdf5 files. One remaining question is: how to deal with missing /event_info, since this is currently required to be able to read an HDF5 file (otherwise raise SystemExit).

We can either "guess" the group_id by taking the first dataset and looking at its IDs (then make a sorted set or so), or even force the HDF5Sink to create a /group_info table, so that it's always available.