Bring isochrone and route buffer mode closer together

Isochrone mode around a point should show the clipped population, and buffer mode should show amenities.

Both UI and implementation-wise, things need to be much more layered. A user can stop at any stage.

1) Pick a single point or a route (or a network) as input
2) Buffer around that by some amount
3) Intersect that buffer with POIs (points) and census data (polygons)