title: Pandas groupby(),agg() - how to return results without the multi index?
body: <p>I have a dataframe:</p>
  
  <pre><code>pe_odds[ [ 'EVENT_ID', 'SELECTION_ID', 'ODDS' ] ]
  Out[67]: 
       EVENT_ID  SELECTION_ID   ODDS
  0   100429300       5297529  18.00
  1   100429300       5297529  20.00
  2   100429300       5297529  21.00
  3   100429300       5297529  22.00
  4   100429300       5297529  23.00
  5   100429300       5297529  24.00
  6   100429300       5297529  25.00
  </code></pre>
  
  <p>When I use groupby and agg, I get results with a multi-index:</p>
  
  <pre><code>pe_odds.groupby( [ 'EVENT_ID', 'SELECTION_ID' ] )[ 'ODDS' ].agg( [ np.min, np.max ] )
  Out[68]: 
                           amin   amax
  EVENT_ID  SELECTION_ID              
  100428417 5490293        1.71   1.71
            5881623        1.14   1.35
            5922296        2.00   2.00
            5956692        2.00   2.02
  100428419 603721         2.44   2.90
            4387436        4.30   6.20
            4398859        1.23   1.35
            4574687        1.35   1.46
            4881396       14.50  19.00
            6032606        2.94   4.20
            6065580        2.70   5.80
            6065582        2.42   3.65
  100428421 5911426        2.22   2.52
  </code></pre>
  
  <p>I have tried using as_index to return the results without the multi_index:</p>
  
  <pre><code>pe_odds.groupby( [ 'EVENT_ID', 'SELECTION_ID' ], as_index=False )[ 'ODDS' ].agg( [ np.min, np.max ], as_index=False )
  </code></pre>
  
  <p>But it still gives me a multi-index.</p>
  
  <p>I can use .reset_index(), but it is very slow:</p>
  
  <pre><code>pe_odds.groupby( [ 'EVENT_ID', 'SELECTION_ID' ] )[ 'ODDS' ].agg( [ np.min, np.max ] ).reset_index()
  
  pe_odds.groupby( [ 'EVENT_ID', 'SELECTION_ID' ] )[ 'ODDS' ].agg( [ np.min, np.max ] ).reset_index()
  Out[69]: 
       EVENT_ID  SELECTION_ID   amin   amax
  0   100428417       5490293   1.71   1.71
  1   100428417       5881623   1.14   1.35
  2   100428417       5922296   2.00   2.00
  3   100428417       5956692   2.00   2.02
  4   100428419        603721   2.44   2.90
  5   100428419       4387436   4.30   6.20
  </code></pre>
  
  <p>How can I return the results, without the Multi-index, using parameters of the groupby and/or agg function. And without having to resort to using reset_index() ?</p>