Collision data - add clustering support

[mvl22]

The collision data suffers from the problem of overlaid points because the resolution of the original data is based on eastings/northings rather than real lat/lons.

There is currently a workaround in the form of jitter=1, but this causes points to move around as the map is redrawn. While this is partly solveable by #8, this is not a full solution.

A plugin such as Leaflet.markercluster is likely to help solve this.
https:/Leaflet/Leaflet.markercluster

May need some API provisioning.