title: how to split dataframe vertically, having N columns in each resulting DF
body: <p>I have the following dataframe:</p>
  <blockquote>
  <pre><code>Date     2017-12-05 2017-12-06 2017-12-15 2017-12-19 2017-12-20  2017-12-21   ....
  time                                                                         
  00:00:00      19.94      21.19      21.88      20.76      21.26      21.26   
  00:15:00      20.29      21.07      21.71      21.79      21.95      21.52   
  00:30:00      21.03      21.25      21.80      22.15      22.26      21.62   
  00:45:00      22.20      21.56      22.77      22.20      22.33      21.91   
  01:00:00      23.25      22.15      23.71      22.31      22.69      21.99   
  01:15:00      23.78      23.33      24.53      22.29      22.82      22.58
  </code></pre>
  </blockquote>
  <p>Columns are dates, and I want to split the dataframe by every 3 date columns. How can I do this?</p>
  <p>Illustration of the final result with one of the resultant dataframes following the split:</p>
  <pre><code>Date      2017-12-05 2017-12-06 2017-12-15 2
       time                                                                         
       00:00:00      19.94      21.19      21.88      
       00:15:00      20.29      21.07      21.71      
       00:30:00      21.03      21.25      21.80      
       00:45:00      22.20      21.56      22.77      
       01:00:00      23.25      22.15      23.71      
       01:15:00      23.78      23.33      24.53
  </code></pre>