Make UrQMD production ready for _air_ showers and comparisons to C7

We need UrQMD at least with valid cross sections as used in C7 for cross-checks and lower energy hadron physics. This is the only simple solution for this purpose.

The current implementation is for sure not OK. We probably mix elastic/inelastic cross sections and we are clearly very different from what C7 does.

It should be quite easy to replicate behaviour of C7. It will be far more complicated in a next step to go to arbitrary target composition, but we will manage if we go the first step first.