title: take mean of data within the same day pandas
body: <p>I have a dataframe <code>df</code> containing date of a measurement and the measurements (<code>duration</code>, <code>km</code>)</p>
  
  <pre><code>df
  Out[20]: 
                            Date duration km
  0   2015-03-28 09:07:00.800001    0      0
  1   2015-03-28 09:36:01.819998    1      2
  2   2015-03-30 09:36:06.839997    1      3
  3   2015-03-30 09:37:27.659997    nan    5
  4   2015-04-22 09:51:40.440003    3      7
  5   2015-04-23 10:15:25.080002    0      nan
  </code></pre>
  
  <p>How can I calculate the average duration and km per day? I would like to take the mean of the rows using groupby and the date...</p>