feat: Consider TTC at stochastic spawning in TrafficAreaAction

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.