title: Parallelize apply after pandas groupby
body: <p>I have used <code>rosetta.parallel.pandas_easy</code> to parallelize <code>apply</code> after <code>groupby</code>, for example:</p>
  
  <pre><code>from rosetta.parallel.pandas_easy import groupby_to_series_to_frame
  df = pd.DataFrame({'a': [6, 2, 2], 'b': [4, 5, 6]},index= ['g1', 'g1', 'g2'])
  groupby_to_series_to_frame(df, np.mean, n_jobs=8, use_apply=True, by=df.index)
  </code></pre>
  
  <p>However, has anyone figured out how to parallelize a function that returns a DataFrame? This code fails for <code>rosetta</code>, as expected.</p>
  
  <pre><code>def tmpFunc(df):
      df['c'] = df.a + df.b
      return df
  
  df.groupby(df.index).apply(tmpFunc)
  groupby_to_series_to_frame(df, tmpFunc, n_jobs=1, use_apply=True, by=df.index)
  </code></pre>