Heavy ions in Pythia8

Angantyr is a new model for collisions involving heavy nuclei in Pythia8. It is inspired by the old Fritiof model, but includes more perturbative physics and a more careful consideration of fluctuations in nucleon-nucleon cross sections, and the related diffractive processes. In the current implementation in Pythia8 parton-level nucleon-nucleon minimum bias events are stacked together and hadronised independently, which means no collective effects are modelled, but the plan is to also include string interaction effects, such as string shoving, colour reconnection and rope hadronisation