Implementation of group behavior model described in 'Pedestrian Group Behavior in a Cellular Automaton' (bib-key: seitz-2014). The basic idea from seitz-2014: Firstly members walking ahead slow down and members falling behind slightly speed up to reach the group. Agents which are added to the topography (before the simulation starts) are assigned to groups directly in the #preLoop(double). Agents which are spawned later on, are assigned to groups in the callback #elementAdded(Pedestrian). The actual behavior of the central group model is implemented in the helper class CentroidGroup.