title: Add number to groups by count of values in Pandas Dataframe
body: <p>I have a pandas Dataframe with a column I would like to group them by pack of 3 rows and then increment an indice on each pack.</p>
  <pre><code>id      protocol    protocol_grp
  1       ISD     ISD1
  2       ISD     ISD1
  3       ISD     ISD1
  4       IRQ     IRQ1
  5       IRQ     IRQ1
  6       IRQ     IRQ1
  7       IRQ     IRQ2
  8       IRQ     IRQ2
  9       IRQ     IRQ2
  10      IRQ     IRQ3
  11      ISD     ISD2
  12      ISD     ISD2
  13      ISD     ISD2
  14      ISD     ISD3
  15      IRQ     IRQ3
  16      IRQ     IRQ3
  17      IRQ     IRQ4
  
  </code></pre>
  <p>The desired output is protocol_grp column.
  What I'd like to be able to do is each time I had 3 same protocols, I increment the indice by 1.</p>
  <p>Hopes this make sense.</p>