This callback class updates the so called agent/pedestrian density every time an agent moves, i.e. whenever this listener is called. Therefore, the listener has to be added in the first place. DensityUpdater#nameAgentDensity is the name of the container of the mesh where the agent density (a Double) value is saved. @param the type of the vertices of the triangulation @param the type of the half-edges of the triangulation @param the type of the faces of the triangulation @author Benedikt Zoennchen