title: Regrouping pandas multiindex columns
body: <p>I am running into an issue where adding new columns to a multiindex column DataFrame causes the new columns to append to the end of the DataFrame.  E.g.:</p>
  
  <pre><code>Group 1         | Group 2        | Group 1 | Group 2 |
  ------------------------------------------------------
  Sub 1  | Sub 2  | Sub 1  | Sub 2 | New Sub | New Sub |
  </code></pre>
  
  <p>Whereas what I want is:</p>
  
  <pre><code>Group 1                   | Group 2                  |
  ------------------------------------------------------
  Sub 1  | Sub 2  | New Sub | Sub 1  | Sub 2 | New Sub |
  </code></pre>
  
  <p>Is there a way to re-group/order my multiindex to do this?  Note- I do not want to re-order the Sub Groups by name, as New Sub needs to go at the end, and alphabetically might not sort correctly.</p>