title: Change certain categorical variables to a unified entry
body: <p>Let's say I have have a dataframe with a column called animals. The entries look as followed:</p>
  <pre><code>'A', 'A', 'A', 'B', 'B', 'B', 'C', 'C', 'E', 'F', 'G', 'H', 'I'.
  </code></pre>
  <p>I want to change the entries 'E', 'F', 'G', 'H' and 'I' to another unified entry called 'D'. What is the best way to transform all these categorical entries into one category?</p>