title: How to select some specific columns in a Mat?
body: <p>In <code>Matlab</code>, if i want select some specified col of a <code>Mat</code> and <strong>at the same time</strong> copy them to another <code>Mat</code>, i write a code like this:</p>
  
  <pre><code>mm = nn(:,[1 3]);
  </code></pre>
  
  <p>how to do like this in OpenCV? </p>