cluster

If the data is a collection of point features, setting this to true clusters the points by radius into groups. Cluster groups become new Point features in the source with additional properties: * cluster: Is true if the point is a cluster * cluster_id: A unique id for the cluster to be used in conjunction with the cluster inspection methods. (TODO which are not implemented yet on GeoJsonSource) * point_count: Number of original points grouped into this cluster * point_count_abbreviated: An abbreviated point count