Default value of jam density does not scale with number of lanes

[toruseo]

Currently, the default jam_density value for Link is set to 0.2. Unless explicitly specified, this value is used regardless of number_of_lanes.

It would be more appropriate to scale the actual jam density by the number of lanes. For example, the default could be 0.1 for a number_of_lanes=1 road and 0.2 for a number_of_lanes=2 road.

However, this change would be significantly breaking, so it will be deferred to v2.0.0.

Also, the current default value of 0.2 veh/m is higher than typical real-world values.