title: Subdivide rows into intervals of 50 and take mean
body: <p>I want to subdivide my dataframe into intervals of 50, then perform the mean of these individual intervals, and then create a new dataframe.</p>
  <pre><code>            2           3           4
      0       20.229517   21.444166   19.528986
      1       19.420929   21.029457   18.895041
      2       19.214857   21.122784   19.228065
      3       19.454653   21.148373   19.249720
      4       20.152334   22.183264   20.149488
      ...     ...         ...         ...
      9995    20.673738   22.252024   21.587578
      9996    21.948563   24.904633   23.962317
      9997    24.318361   27.220770   25.322933
      9998    24.570177   26.371695   23.503048
      9999    23.274368   25.145500   22.028172
      10000 rows × 3 columns
  </code></pre>
  <p>That is a 200 x 3, with the mean values. I would  like to do this in pandas if possible. Thx in advance!</p>