title: Create two aggregate columns by Group By Pandas
body: <p>I'm new to DataFrames and I want to group multiple columns and then sum and keep a count on the last column. e.g.</p>
  
  <pre><code>s = pd.DataFrame(np.matrix([[1, 2,3,4], [3, 4,7,6],[3,4,5,6],[1,2,3,7]]), columns=['a', 'b', 'c', 'd'])
  
     a  b  c  d
  0  1  2  3  4
  1  3  4  7  6
  2  3  4  5  6
  3  1  2  3  7
  </code></pre>
  
  <p>I want to group on <code>a</code>, <code>b</code> and <code>c</code> but then sum on <code>d</code> and count the elements within the group. 
  I can count by </p>
  
  <pre><code>s = s.groupby(by=["a", "b", "c"])["d"].count()
  
      a  b  c
      1  2  3    2
      3  4  5    1
            7    1
  </code></pre>
  
  <p>And I can sum by</p>
  
  <pre><code>s = s.groupby(by=["a", "b", "c"])["d"].sum()
  
  a  b  c
  1  2  3    11
  3  4  5     6
        7     6
  </code></pre>
  
  <p>However I want to combine it such that The resulting dataframe has both the sum and count columns. </p>
  
  <pre><code>    a  b  c   sum    count
      1  2  3    11     2
      3  4  5     6     1
            7     6     1
  </code></pre>