There seems to be a front-end library that can do the clustering. `leaflet.markercluster.js`, https://github.com/Leaflet/Leaflet.markercluster
 Should work for the nearby feature or for user added feature collections on maps.
 Nearby and normal markers should be done separately, the code is different.
Open design questions:
- How will the numbered markers and grouped-count markers be distinguished from one another?
- They will be styled differently.
- Do we respect colors, e.g. group green with green and blue with blue?
- No. The pin groups will always have the same color, regardless of the colors of grouped pins within.
- When we’re highlighting a specific marker, should that be kept out of the clustering? (and styled in some special way?)
- Make clustering an optional setting? For the reader? Per map?
- No, this will not be a setting.