title: Sum column based on another column in Pandas DataFrame
body: <p>I have a pandas DataFrame like this:</p>
  
  <pre><code>&gt;&gt;&gt; df = pd.DataFrame({'MONTREGL':[10,10,2222,35,200,56,5555],'SINID':['aaa','aaa','aaa','bbb','bbb','ccc','ccc'],'EXTRA':[400,400,400,500,500,333,333]})
  &gt;&gt;&gt; df
     MONTREGL SINID EXTRA
  0        10   aaa   400
  1        10   aaa   400
  2      2222   aaa   400
  3        35   bbb   500
  4       200   bbb   500
  5        56   ccc   333
  6      5555   ccc   333
  </code></pre>
  
  <p>I want to sum the column <code>MONTREGL</code> for each groupby <code>SINID</code>...</p>
  
  <p>So I get 2242 for aaa and so on... ALSO I want to keep the value of column <code>EXTRA</code>.</p>
  
  <p>This is the expected result:</p>
  
  <pre><code>   MONTREGL SINID EXTRA
  0      2242   aaa   400
  1       235   bbb   500
  2      5611   ccc   333
  </code></pre>
  
  <p>Thanks for your help in advance!</p>