Smart traffic spawner

Often we don't have a realistic travel demand model, or we don't even care / want one. Instead, we have some more narrow definition of traffic that we'd like to see.

- in the LTN tool, have cars backed up along the perimeter road, and show a few rat-running through
- for things like the Aurora workshop demo, just sending lots of north/south through-traffic, maybe with a background level elsewhere
- in the bike network tool and from Samir's feedback, just a way to see some normal movement through a particular area we're zoomed in on. Show lots of cars before changes, and bikes dodging traffic in newly built lanes.

The point of these isn't realism at all; it's context-sensitive demonstration.