title: How to divide the sum with the size in a pandas groupby
body: <p>I have a dataframe like</p>
  
  <pre><code>  ID_0 ID_1  ID_2
  0    a    b     1
  1    a    c     1
  2    a    b     0
  3    d    c     0
  4    a    c     0
  5    a    c     1
  </code></pre>
  
  <p>I would like to groupby ['ID_0','ID_1'] and produce a new dataframe which has the sum of the ID_2 values for each group divided by the number of rows in each group.</p>
  
  <pre><code>grouped  = df.groupby(['ID_0', 'ID_1'])
  print grouped.agg({'ID_2': np.sum}), "\n", grouped.size()
  </code></pre>
  
  <p>gives</p>
  
  <pre><code>           ID_2
  ID_0 ID_1
  a    b        1
       c        2
  d    c        0
  ID_0  ID_1
  a     b       2
        c       3
  d     c       1
  dtype: int64
  </code></pre>
  
  <p>How can I  get the new dataframe with the np.sum values divided by the size() values?</p>