title: How to get GeoJSON properties to appear in openlayers 3 popup when using CLUSTERS?
body: <p>I've built a relatively simple openlayers map and I'm attempting to get my geoJSON properties to appear in my on-click popup but no matter what I try, it either returns the geometry name <code>object Object</code> or <code>Undefined</code>. I think openlayers stores cluster data as an array of features rather than multiple individual features? Here is the code for my on-click function:</p>
  
  <pre><code>var
     container = document.getElementById('popup'),
     content_element = document.getElementById('popup-content'),
     closer = document.getElementById('popup-closer');
  
  closer.onclick = function() {
  overlay.setPosition(undefined);
  closer.blur();
  return false;
  };
  
  var overlay = new ol.Overlay({
  element: container,
  positioning: 'bottom-center',
  stopEvent: false
  });
  
  map.addOverlay(overlay);
  // display popup on click
  map.on('click', function(evt) {
  var feature = map.forEachFeatureAtPixel(evt.pixel,
    function(feature, layer) {
      return feature;
    });
  if (feature) {
  var geometry = feature.getGeometry();
      var coord = geometry.getCoordinates();
  
      var content =coord;
  //------------------------------------------------------------------------------
      content += '&lt;h5&gt;' + feature.get('MMSI') + '&lt;/h5&gt;';
  //--------^^^^^^^ This does not work for clusters ^^^^^^------------------------
  //------------------------------------------------------------------------------
      content_element.innerHTML = content;
      overlay.setPosition(coord);
  } else {
  
  }
  </code></pre>
  
  <p>I don't need the properties of the actual clusters but I do need the properties of a single point(once zoomed in and the clusters have separated into individual points). Any help is much appreciated!</p>