title: how to convert monthly data to quarterly in pandas
body: <p>I have monthly data.  I want to convert it to "periods" of 3 months where q1 starts in January.  So in the example below, the first 3 month aggregation would translate into start of q2 (desired format: 1996q2). And the data value that results from mushing together 3 monthly values is a mean (average) of 3 columns.   Conceptually, not complicated.  Does anyone know how to do it in one swoop? Potentially, I could do a lot of hard work through looping and just hardcode the hell out of it, but I am new to pandas and looking for something more clever than brute force.</p>
  
  <pre>
  1996-04   1996-05 1996-06  1996-07 .....
  25          19       37      40
  </pre>
  
  <p>So I am looking for:</p>
  
  <pre>
  1996q2  1996q3   1996q4  1997q1  1997q2 .....
   avg      avg      avg     ...     ...
  </pre>