Generation of Heavy Ion collisions in PYTHIA with inclusion of collective effects from string interactions.

The work follows from the string-shoving in Angantyr of PYTHIA8. This includes calculating non-trivial space-time overlap of Lund strings and the resulting repulsion as an effort to understand their dynamics in heavy-ion collisions. By the end of this sub-project, the aim is to have the physics of the Lund string model defined for heavy-ion collisions, coupled with colour reconnection.