Feature: Add Leaflet Controls to position based on user or marker aggregate

[fpapado]

Having every addition to the map call setView() seems silly in terms of performance, and I also worry that we are relying on browser timing when we do that. I wonder if MutationObserver allows us to run a command after each action has completed.

[fpapado]

This turned out to be tricky, because the mutations are only done processing after the aggregate of children nodes has been added. We could track those with a Promise (initialChildrenLoads), but I'm a bit wary of the complexity. Perhaps a better feature is to add controls for the user, in the form of Geolocation and Marker buttons.