title: Calculate minimum value for each column of multi-indexed DataFrame in pandas
body: <p>I have a multi-indexed DataFrame with the following structure:</p>
  
  <pre><code>         metric1                 metric2            
       experiment1 experiment2 experiment1 experiment2
  run1         1.2         1.5         0.2         0.9
  run2         2.1         0.7         0.4         4.3
  </code></pre>
  
  <p>How can I calculate minimum (maximum, mean, etc.) value for each column and get DataFrame like this:</p>
  
  <pre><code>         metric1                 metric2            
       experiment1 experiment2 experiment1 experiment2
  run1         1.2         1.5         0.2         0.9
  run2         1.6         0.9         0.3         3.1
  run3         2.1         0.7         0.4         4.3
  min          1.2         0.7         0.2         0.9
  max          2.1         1.5         0.4         4.3
  </code></pre>