title: Sum columns by level in a pandas MultiIndex DataFrame
body: <p>I have my df with multi-index columns. All of my values are in float, and I want to merge values with in first level of multi-index. Please see below for detail.</p>
  
  <pre><code>first        bar                 baz                 foo   
  second       one       two       one       two       one    
  A       0.895717  0.805244  1.206412  2.565646  1.431256    
  B       0.410835  0.813850  0.132003  0.827317  0.076467    
  C       1.413681  1.607920  1.024180  0.569605  0.875906 
  
  first        bar                 baz                 foo   
  
  A       (0.895717+0.805244) (1.206412+2.565646)  1.431256    
  B       (0.410835+0.813850) (0.132003+0.827317)  0.076467    
  C       (1.413681+1.607920) (1.024180+0.569605)  0.875906 
  </code></pre>
  
  <p>The values are actually added (I just didn't feel like doing all this :)). Bottom line is that I just want to level-up(higher level I guess) and within the index, add all the values. Please let me know a good way to do this. Thank you!</p>