hadronic interactions with Pythia

Follow up to #620 (closed)

added Hadronic label

created branch 710-hadronic-interactions-with-pythia to address this issue

mentioned in merge request !649 (closed)

changed milestone to %air showers with pythia

assigned to @riehn

mentioned in issue #712

mentioned in merge request !676 (merged)

mentioned in commit 42d008d6

closed with merge request !676 (merged)