First implementation of HGPythia

[jackal1-66]

The integration is based on the original code in https://github.com/abaty/HGPythia, with a few fixes that have been suggested as well there (two opened PRs).
This was requested by @ddobrigk

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[ddobrigk]

Thanks a lot @jackal1-66, this looks very good! I see you also propagated the relevant information (Ncoll, Npart, etc) to the event header, which I think will mean that it will be accessible for analysis as it should: great! I would say we can go ahead and give it a go in the MCGEN / OTF MC trains :-)