title: Convert hour:minute:second (HH:MM:SS) string to proper time class
body: <p>In the following data frame the 'time' column is <code>character</code> in the format <code>hour:minute:second</code></p>
  <pre><code>id &lt;- c(1, 2, 3, 4)
  time &lt;- c(&quot;00:00:01&quot;, &quot;01:02:00&quot;, &quot;09:30:01&quot;, &quot;14:15:25&quot;)
  df &lt;- data.frame(id, time)
  </code></pre>
  <p>How can I convert 'time' column to a dedicated time class, so that I can perform arithmetic calculations on it?</p>