title: pandas groupby without turning grouped by column into index
body: <p>The default behavior of pandas groupby is to turn the group by columns into index and remove them from the list of columns of the dataframe. For instance, say I have a dataFrame with these columns </p>
  
  <pre><code>col1|col2|col3|col4
  </code></pre>
  
  <p>if I apply a groupby say with columns <code>col2</code> and <code>col3</code> this way</p>
  
  <pre><code>df.groupby(['col2','col3']).sum()
  </code></pre>
  
  <p>The dataframe <code>df</code> no longer has the <code>['col2','col3']</code> in the list of columns. They are automatically turned into the indices of the resulting dataframe. </p>
  
  <p>My question is how can I perform groupby on a column and yet keep that column in the dataframe?  </p>