title: Pandas take average over regular intervals of columns
body: <p>Given a pandas dataframe like such:</p>
  
  <pre><code>    Col_1 Col_2 Col_3 Col_4 Col_5 Col_6
  0     1     2     3     4     5     6
  1     7     8     9     10    11    12
  </code></pre>
  
  <p>I'd like to take the average over subsets of columns, defined by some given interval. Say the interval is two, then I want to take the average of Col_1 and Col_2, Col_3 and Col 4 as well as Col_5 and Col6. Or given an interval of 3, I'd like the output to be</p>
  
  <pre><code>    First_Avg  Second_Avg
  0      2          5
  1      8          11
  </code></pre>
  
  <p>It is to be assumed that I cannot go off the naming of my columns.
  Code:</p>
  
  <pre><code> import pandas as pd
   df = pd.DataFrame(columns =['Col_'+str(n) for n in range(1,7)], data =np.arange(12).reshape(2,6))
  </code></pre>