title: Read yml file with OpenCV
body: <p>With OpenCV, I want to find keypoints in different images and save them on my hard disk. This works very well for the saving part. To save the keypoints I use:</p>
  
  <pre><code>Mat it;
  it = imread( "pic1.jpg", IMREAD_GRAYSCALE);
  vector&lt;KeyPoint&gt; keypoints;
  detector.detect( (it), keypoints );
  FileStorage fs("keypoint1.yml", FileStorage::WRITE);
  write( fs , "keypoint", keypoints );
  fs.release();
  </code></pre>
  
  <p>When I try to read the file again with:</p>
  
  <pre><code>vector&lt;KeyPoint&gt; keypoint1s;
  FileStorage fs2("keypoint1.yml", FileStorage::READ);
  FileNode kptFileNode = fs2["keypoint1"];
  read( kptFileNode, keypoint1s );
  fs2.release();
  </code></pre>
  
  <p>If I do it like that, "keypoint1s" is empty. What is wrong?</p>