title: Merge MultiIndex columns together into 1 level
body: <p>Here's some data from another question:</p>
  
  <pre><code>date       type       value
  1/1/2016   a          1
  1/1/2016   b          2
  1/1/2016   a          1
  1/1/2016   b          4
  1/2/2016   a          1
  1/2/2016   b          1
  </code></pre>
  
  <p>Run this line of code:</p>
  
  <pre><code>x = df.groupby(['date', 'type']).value.agg(['sum', 'max']).unstack()
  </code></pre>
  
  <p><code>x</code> should look like this:</p>
  
  <pre><code>         sum    max   
  type       a  b   a  b
  date                  
  1/1/2016   2  6   1  4
  1/2/2016   1  1   1  1
  </code></pre>
  
  <p>I want to combine the columns on the upper and lower level to get this:</p>
  
  <pre><code>           sum_a  sum_b   max_a  max_b
  date                  
  1/1/2016   2       6        1       4
  1/2/2016   1       1        1       1
  </code></pre>
  
  <p>Is there an easy way to do this?</p>