title: MarkerClusterer is marker in cluster?
body: <p>I put markers into clusters:</p>
  
  <pre><code>var markerClusterer = new MarkerClusterer(map, markers, {
      zoomOnClick : false,
      maxZoom : 13,
      gridSize : 100
  });
  </code></pre>
  
  <p>And i have 15 markers. 10 of these are in clusters in the map. How to determine if the marker is in a clusters.</p>
  
  <pre><code>var clusteredMarkers = markerClusterer.getTotalMarkers();
  for(i = 0; i &lt; clusteredMarkers.length; i++) {
      if(isInCluster((clusteredMarkers[i])) {
          clusteredMarkers[i].infobox.close();        
      }
  }
  </code></pre>
  
  <p>How to define a function like isInCluster(marker) that infobox is open in only markers, that are not in any cluster (i.e. 5 infoboxes have to be visible)?</p>