title: How to unfold lists in DataFrame columns?
body: <p>I have the following DataFrame <code>df</code>:</p>
  
  <pre><code>center    status     devices
  1     Green      [d1, d2]
  1     Green      [d5, d1, d2]
  2     Green      []
  3     Green      [d5, d6]
  </code></pre>
  
  <p>I need to unfold lists in the column <code>devices</code>. The goal is to group data by <code>center</code> and <code>device</code>, and then count the number of observations per group.</p>
  
  <p>The expected result would be the following one:</p>
  
  <pre><code>center   device   count
  1        d1       2
  1        d2       2
  1        d5       1
  3        d5       1
  3        d6       1
  </code></pre>