title: convert index to column pandas dataframe
body: <p>I have following pandas dataframe:</p>
  
  <pre><code>                    |     id    |  LocTime        |ZPos   | XPos
  datetime            |               
  2017-01-02 00:14:39 |20421902611|   12531245409231| 0     | -6              
  2017-01-02 00:14:40 |30453291020|   28332479673070| 0     | -2  
  </code></pre>
  
  <p>I want to convert datetime index to column of the data frame. 
  I tried <code>df.reset_index(level=['datetime'])</code> but the result does not change.
  any idea?            </p>