feat: Set model3d for traffic light

For traffic light entities the model_reference attribute wasn't set in the OSI GT, yet.
Since in the entity properties the whole traffic light entity has only one model_3d attribute but we need one model_reference for each OSI::TrafficLight, the solution is (analogous to switching the state) that the user must specify the same amount of 3d models as there are bulbs in the traffic light or no model3d (empty string) in the model_3d attribute. The 3d models have to be separated by a comma.