Pythia8 UserHook to shower HDF5 input files

Adds HDF5 input compatibility using a custom UserHook for the Athena Pythia8 interface that allows reading of externally provided HDF5 input files in order to pass the MC events from these files through the Pythia8 parton shower for fragmentation and hadronisation using traditional grid-computing resources. This UserHook aims to better exploit HPC resources for event generation in the future and reach unprecedented precision in multi-leg calculations. Documentation for the QT can be found here: https://cds.cern.ch/record/2749837