title: How to move pandas data from index to column after multiple groupby
body: <p>I have the following pandas dataframe:</p>
  <pre><code>token    year    uses  books
    386   xanthos  1830    3     3
    387   xanthos  1840    1     1
    388   xanthos  1840    2     2
    389   xanthos  1868    2     2
    390   xanthos  1875    1     1
  </code></pre>
  <p>I aggregate the rows with duplicate <code>token</code> and <code>years</code> like so:</p>
  <pre><code>dfalph = dfalph[['token','year','uses','books']].groupby(['token', 'year']).agg([np.sum])
  dfalph.columns = dfalph.columns.droplevel(1)
  
                 uses  books
  token    year       
  xanthos  1830    3     3
           1840    3     3
           1867    2     2
           1868    2     2
           1875    1     1
  </code></pre>
  <p>Instead of having the 'token' and 'year' fields in the index, I would like to return them to columns and have an integer index.</p>