title: group by in group by and average
body: <p>I have a dataframe like this:</p>
  <pre><code>cluster  org      time
     1      a       8
     1      a       6
     2      h       34
     1      c       23
     2      d       74
     3      w       6 
  </code></pre>
  <p>I would like to calculate the average of time per org per cluster.</p>
  <p>Expected result:</p>
  <pre><code>cluster mean(time)
  1       15 #=((8 + 6) / 2 + 23) / 2
  2       54 #=(74 + 34) / 2
  3       6
  </code></pre>
  <p>I do not know how to do it in Pandas, can anybody help?</p>