title: Find unique values for each column
body: <p>I am looking to find the unique values for each column in my dataframe. (Values unique for the whole dataframe)</p>
  
  <pre><code>        Col1         Col2            Col3
  1        A             A               B
  2        C             A               B
  3        B             B               F
  </code></pre>
  
  <p>Col1 has C as a unique value, Col2 has none and Col3 has F.</p>
  
  <p>Any genius ideas ? thank you !</p>