Customize disableClusteringAtZoom level by transit mode #99
Description
A Leaflet MarkerCluster layer is initialized with a disableClusteringAtZoom parameter, with a value between 0 (all the way zoomed out) and 19 (all the way zoomed in). I set it at 16, which is roughly the level at which you should know when bus stops along the same street are distinct.
However, I suspect that the proper clustering level is dependent on mode of transit: you want subway stations to be fully declustered at a more zoomed out level (perhaps 11 or 12) and commuter rail to decluster even further out than that (maybe 9 or 10)
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This suggests we might actually want initialize multiple MarkerClusters on the map: one per mode of transit, as determined by the GTFS route_type attribute (which exists in our backend). Potential follow-up issue to that: let user toggle on/off the MarkerClusters by mode as desired (display the subway stops but not the buses, e.g.)