TrafficAreaAction: Consider TTC at stochastic spawning

New entites use a sampled time gap (user defined parameter tgap) and initial velocity (user defined parameter velocity). As those parameters alone do not guarantee a crash-free, this commit introduces a basic TTC robustness check for newly spawned entities, so that the likelyhood for collisions is reduced.

The TTC check includes reaction time and an assumed deceleration under the restriction that the opponent is moving at constant velocity.