Map plots tracks, not density

[mrhheffernan]

Map is really a plotter of tracks, not densities. Existing heatmaps don’t do a good job of capturing activity data.

Proposed fix: use h3 indices at highest resolution, alternately another geospatial indexing system.

Ideal fix would be something like processing sections at a time, generating tiles with something like a numpy histogram, and colouring by density. This is more involved and unlikely to be achievable in a lightweight way.