title: Three.js How to use quaternion to rotate camera
body: <p>In Three.js, I would like to use THREE.quaternion to make the camera object rotate to the selected object.</p>
  
  <p>I did search the web but found no example/demo or document about how to use this quaternion class.</p>
  
  <p>I try my luck with following code:</p>
  
  <pre><code>    camera = new THREE.PerspectiveCamera( 40, window.innerWidth / window.innerHeight, 1, 10000 );
      camera.position.y = 10;
      camera.position.z = 0;
      camera.position.x = radious;
      camera.useQuaternion = true;
  
     // I did use the TrackballTrackballControls. Maybe it causes the problem so I put it here
      controls = new THREE.TrackballControls( camera, document.getElementById(_canvasElement) );
  
      // function to make the camera rotate to the object
      function focusOn3DObject(obj){  
          obj.useQuaternion = true;
          obj.quaternion = new THREE.Quaternion(obj.position.x, obj.position.y, obj.position.z, 1);
  
          var newQuaternion = new THREE.Quaternion();
          THREE.Quaternion.slerp(camera.quaternion, obj.quaternion, newQuaternion, 0.07);
          camera.quaternion = newQuaternion;
      }
  </code></pre>
  
  <p>But it doesn't work. Did I miss something?
  Please help. Thanks in advance.</p>