Path cost tuner

Writing down a half-baked idea. How do you tune the cost function for pathfinding? Examples could be parameter tuning to make it worth going a little farther to get to a protected bike trail. Or maybe even a quick estimate without simulation of how many people might alter their route if you modify lanes.

The idea is to take all of the `PathRequests` that come from a `Scenario`, gather the throughput counts for all of them, make the lane edit or tune a parameter, calculate the difference in throughput, and display the result as a heatmap.