Additonal details on the `RouteField` class

[alafage]

Hi @melights,

Thank you for open-sourcing your work!
I was skimming through your vector representation and wondered whether you could explain more about the attributes ROAD_WIDTH0 and ROAD_WIDTH1 in the RouteField class. What are they? How can I use them to build polygons/lanelets from RouteField?

Best,