Add model_reference attribute to RoadMarking (#874)

[arauschert]

Reference to a related issue in the repository

#874

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Adding the model_reference attribute to the RoadMarking class.
This was missing and is now consistent with the other classes TrafficLight, MovingObject...

Backward-compatible, as added as optional and at the end (attribute #6).

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[thomassedlmayer]

Looks good to me. I don’t think there are any special cases for specific road marking types as discussed in #874. All road marking types must conform to the base dimensions regardless of their type. So the model reference role should be consistently implementable for all types.

My only remark for road marking model placement is that road markings' base objects are positioned differently from other OSI objects: their local z-axis corresponds to how the marking is placed on the xy-plane of the lane (see OSI documentation for more details), rather than pointing upwards as it does for other objects. But as the model reference use is left to the implementer, I guess this just plays a role when you're trying to be consistent (in one way or another) in your model reference implementation.