title: Create pandas column with tree information
body: <p>I've a pandas df with the column '<code>Tree</code>', and I'd like to create the column '<code>Leaf</code>' which only contains <code>integers</code>, differentiating if that taxonomy belongs to the same final leaf:</p>
  <pre><code>'index'    'Tree'
  0          'woman | watches | steal'
  1          'woman | dresses | short-sleve'
  2          'woman | watches | steal'
  3          'woman | dresses | short-sleve'
  4          'woman | dresses | long-sleve'     
  </code></pre>
  <p>Output:</p>
  <pre><code>'index'    'Tree'                            'Leaf' 
  0          'woman | watches | steal'          0
  1          'woman | dresses | short-sleve'    1
  2          'woman | watches | steal'          0 
  3          'woman | dresses | short-sleve'    1
  4          'woman | dresses | long-sleve'     2 
  </code></pre>
  <p>Any suggestions how to do it fast?</p>