Try voronoi for visualizing cells

[Cells](https://cnt.scot/user_guide?lang=en#understanding-the-map-cells) are currently visualized first by coloring each reachable road:

Then discretizing into grids and "flooding" evenly:

And then using marching squares contouring to clean up the shape.

Just to experiment with the new Voronoi support in https://github.com/georust/geo/pull/1487/, it'd be neat to see if the cell shapes can be more intuitive. We want to find the cell areas closest group of roads (so, a `MultiLineString` input as one "site"). Voronoi faces are found for each input point right now, but maybe if we then merge adjacent cells that all belong to the same input group, the final result looks decent?

https://gis.stackexchange.com/questions/104631/creating-voronoi-diagram-from-line-segments describes other approaches