Add a vehicle counts layer. #318

[dabreegster]

Demo: https://acteng.github.io/atip/vehicle_counts/browse.html

https://github.com/acteng/atip/assets/1664407/2f8c5780-dcdb-4a95-9055-177f21ffe024

This should be enough to start, but plenty of next steps:

• icons instead of points
• display all points when unzoomed (or at least some kind of clustering or heatmap to indicate where the points are)
• adjust the buckets -- not seeing the higher values much

[dabreegster]

Started #326 to track problems like few points at low zoom