title: How to sum time in a dataframe
body: <p>I have a dataframe of time data in the format</p>
  
  <pre><code>hh:mm:ss
  hh:mm:ss
  </code></pre>
  
  <p>(type string)</p>
  
  <p>I need to be able to sum the values (to acquire total time) in a few of the columns.  I'm wondering if anyone has any recommendations on the best way to do this and get the sum in the same format.</p>