title: Take the sum of every N rows in a pandas series
body: <p>Suppose </p>
  
  <p><code>s = pd.Series(range(50))</code></p>
  
  <pre><code>0      0
  1      1
  2      2
  3      3
  ...
  48     48
  49     49
  </code></pre>
  
  <p>How can I get the new series that consists of sum of every n rows?</p>
  
  <p>Expected result is like below, when n = 5;</p>
  
  <pre><code>0      10
  1      35
  2      60
  3      85
  ...
  8      210
  9      235
  </code></pre>
  
  <p>If using loc or iloc and loop by python, of course it can be accomplished, however I believe it could be done simply in Pandas way.</p>
  
  <p>Also, this is a very simplified example, I don't expect the explanation of the sequences:). Actual data series I'm trying has the time index and the the number of events occurred in every second as the values.</p>