This function allows to create Markers which are grouped depending on their distances on the current view.
This feature is provided by gmap3 and is not a native one.

Parameters Cluster options Options Array of MarkerOptions Callback function to evaluate if an array of Marker(s) have to be grouped or not.
If so, the function have to return an overlay options.

Cluster Class

The Cluster object returned by this function allows to manipulate the clustering feature.

Return the Marker list. Return the current ClusterOverlay. Enable the clustering feature. Disable the clustering feature. Add a Marker. Remove a Marker. Set a filtering function or remove if fn is null.
This function is called for each Marker.
If it returns true, the Marker is shown else, it is hidden.

A ClusterOverlay contains one or more Markers.

Cluster instance. Array of Marker Node container of the google map. OverlayView of the group. Returns the LatLngBounds of the overlay.