[GroupModel] Branch "group_models": some group members stop suddenly

In attached scenario OPMOPS-Demo1.scenario, some group members stop suddenly. For instance, from time step 300 on.

The affected classes are:

• CentroidGroupSpeedAdjuster: this class is used by PedestrianOSM's getDesiredSpeed() method.
• UpdateSchemeEventDriven: this class sets time of next step. See following snippet:

pedestrian.setDurationNextStep(pedestrian.getStepSize() / pedestrian.getDesiredSpeed());
...
pedestrian.setTimeOfNextStep(pedestrian.getTimeOfNextStep() + pedestrian.getDurationNextStep());

But, if getDesiredSpeed() returns a small number (close to zero) the next step will be made only in the far future (and the pedestrians won't make any step again).

As discussed with @BZoennchen on July 11, 2018, we should not use a speed adjuster to enforce a waiting behavior of a pedestrian. Instead, we should use events (like wait or go). @hm-kleinmei will think about an event concept here.