Sorcha cannot read HDF5 files in the format it writes them

[astronomerritt]

Rather than simply append to an existing dataset, PPOutput.PPOutWriteHDF5() creates a new dataset within the file for every chunk, with the key being the index of the last item in the chunk.

This causes problems when creating HDF5 files of the Sorcha-generated ephemeris, as the HDF5DataReader class can't read this format back in.

I already have a solution for this - you can use pd.HDFStore to append to existing datasets/tables within a HDF5 file. I just need to implement it and make sure it works with HDF5DataReader.