/**
  * Get the ClusterItem from a marker
  *
  * @param marker which you will obtain its ClusterItem
  * @return a ClusterItem from a marker or null if it does not exists
  */
 public T getClusterItem(Marker marker) {
   return mMarkerCache.get(marker);
 }
 /**
  * Get the marker from a ClusterItem
  *
  * @param clusterItem ClusterItem which you will obtain its marker
  * @return a marker from a ClusterItem or null if it does not exists
  */
 public Marker getMarker(T clusterItem) {
   return mMarkerCache.get(clusterItem);
 }