title: Change row order in a matrix/dataframe
body: <p>I need to change/invert rows in my data frame, not transposing the data but moving the bottom row to the top and so on. If the data frame was:</p>
  
  <pre><code>1 2 3 
  4 5 6
  7 8 9
  </code></pre>
  
  <p>I need to convert to</p>
  
  <pre><code>7 8 9
  4 5 6
  1 2 3
  </code></pre>
  
  <p>I've read about <code>sort()</code> but I don't think it is what I need or I'm not able to find the way.</p>