title: pandas get average of a groupby
body: <p>I am trying to find the average monthly cost per user_id but i am only able to get average cost per user or monthly cost per user. </p>
  
  <p>Because i group by user and month, there is no way to get the average of the second groupby (month) unless i transform the groupby output to something else.</p>
  
  <p>This is my df:</p>
  
  <pre><code>     df = { 'id' : pd.Series([1,1,1,1,2,2,2,2]),
              'cost' : pd.Series([10,20,30,40,50,60,70,80]),
              'mth': pd.Series([3,3,4,5,3,4,4,5])}
  
     cost  id  mth
  0    10   1    3
  1    20   1    3
  2    30   1    4
  3    40   1    5
  4    50   2    3
  5    60   2    4
  6    70   2    4
  7    80   2    5
  </code></pre>
  
  <p>I can get monthly sum but i want the average of the months for each user_id. </p>
  
  <pre><code>df.groupby(['id','mth'])['cost'].sum()
  
  id  mth
  1   3       30
      4       30
      5       40
  2   3       50
      4      130
      5       80
  </code></pre>
  
  <p>i want something like this:</p>
  
  <pre><code>id average_monthly
  1 (30+30+40)/3
  2 (50+130+80)/3
  </code></pre>