title: Input missed values with mean of nearest neighbors in column
body: <p>I have a DataFrame:</p>
  
  <pre><code>df = pd.DataFrame(data=[676, 0, 670, 0, 668], index=['2012-01-31 00:00:00','2012-02-29 00:00:00',
                                                       '2012-03-31 00:00:00','2012-04-30 00:00:00',
                                                       '2012-05-31 00:00:00'])  
  df.index.name = "Date"
  df.columns = ["Number"]
  </code></pre>
  
  <p>Which looks like:</p>
  
  <pre><code>              Number
  Date    
  2012-01-31 00:00:00 676
  2012-02-29 00:00:00 0
  2012-03-31 00:00:00 670
  2012-04-30 00:00:00 0
  2012-05-31 00:00:00 668
  </code></pre>
  
  <p>How can i input 2nd and 4th values with (676+670)/2 and (670+668)/2 correspondinly?</p>
  
  <p>I can save values as <code>np.array</code> and imput them in array, but that's rediculous!</p>