Add relative model path to objects

I started a PR (#284) in order to add 3D model paths to static objects and dynamic objects. This extension can be used to implement sensor models that rely on more sophisticated three-dimensional description of the environment. Please feel free to discuss this idea.

After considering potential inconsistencies and/or ambiguousities in the implementation of the concept I postpone its implementation to OSI 4.0.0. A revision of the concept of object types could be helpful here.