title: Merging DataFrame row values into string by level
body: <p>If I have the following DataFrame...</p>
  
  <pre><code>           code
  player_id      
  223336        4
  223336        5
  223336        4
  225987        2
  225987        3
  225987        4
  </code></pre>
  
  <p>How can I merge the "code" column into a string so the result would look like...</p>
  
  <pre><code>             code
  player_id      
  223336        454
  225987        234
  </code></pre>
  
  <p>Thanks!</p>