title: pandas dataframe groupby sum index
body: <p>I have a dataframe, I want to </p>
  
  <pre><code>    FROM:  
               dow    yield
             0    F   2
             1    F   3
             2    M   4
             3    M   6
             4    TH  7
      TO:
  
            dow    ysum
        0     F   5
        1     M   10
        2     TH  7
  </code></pre>
  
  <p>butI got this :</p>
  
  <pre><code>               |yield
           -------------
            dow  |
           -------------
             F   |5
             M   |10
             TH  |7
  </code></pre>
  
  <p>This is how I did it:</p>
  
  <pre><code>    d1=['F','F','M','M','TH']
      d2=[2,3,4,6,7]
      d = {'dow': d1, 'yield': d2}
      df = pd.DataFrame(data=d, index=None)
      df1= df.groupby('dow').sum()
  </code></pre>
  
  <p>How could get result use dow as a column in stead of index?</p>