title: compatibility issue with contourArea in openCV 3
body: <p>I am trying to do a simple area calculation of contours I get from findContours.
  My openCv version is 3.1.0</p>
  
  <p>My code is:</p>
  
  <pre><code>cc = cv2.findContours(im_bw.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  cv2.contourArea(cc[0])
  
  error: 'C:\\builds\\master_PackSlaveAddon-win32-vc12-static\\opencv\\modules\\imgproc\\src\\shapedescr.cp...: error: (-215) npoints &gt;= 0 &amp;&amp; (depth == CV_32F || depth == CV_32S) in function cv::contourArea\n'
  </code></pre>
  
  <p>Cant seem to solve it, I have a feeling its just typecasting altough I expect the findContours result to match the type of contourArea</p>
  
  <p>Thanks :)</p>
  
  <p>EDIT: turns out I need to take the 2nd argument of findContours</p>
  
  <pre><code> im2, cc, hierarchy = cv2.findContours(im_bw.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  </code></pre>