hadronic interactions with pythia8

Pythia8 now also has heavy ion interactions so full air showers can be done. First step is with proton target only.

Nice-to-have but not essential.

mentioned in issue #54 (closed)

mentioned in issue #150 (closed)

indeed!

created merge request !90 (merged) to address this issue

mentioned in merge request !90 (merged)

closed via merge request !90 (merged)

mentioned in commit 1899c726