title: Saving KeyPoint as String and converting back to KeyPoint
body: <p>I would like to cache <code>KeyPoint</code> in a JSON file and then retrieve them later for use in a <code>FlannBasedMatcher</code>. Is there a way to convert the <code>KeyPoint</code> to something like an array of strings or floats that could be stored and then retreived from a JSON file? I think this should be ok for the descriptors since they just look like an array of ints.</p>
  
  <p><strong>COMPUTING KEYPOINTs</strong></p>
  
  <pre><code>kp2, des2 = brisk.detectAndCompute(img2, None)
  </code></pre>
  
  <p><strong>MATCHER</strong></p>
  
  <pre><code>matches = flann.knnMatch(des1,des2,k=2)
  </code></pre>