Should contain code that creates DOM elements for the overlay, adds them
to map panes where they should belong and puts listeners on relevant map events.
Called on map.addLayer(layer).
Should contain all clean up code that removes the overlay's elements from
the DOM and removes listeners previously added in onAdd. Called on map.removeLayer(layer).
Should contain code that creates DOM elements for the overlay, adds them to map panes where they should belong and puts listeners on relevant map events. Called on map.addLayer(layer).