title: Get all combinations of elements from two lists?
body: <p>If I have two lists </p>
  
  <pre><code>l1 = [ 'A', 'B' ]
  
  l2 = [ 1, 2 ]
  </code></pre>
  
  <p>what is the most elegant way to get a pandas data frame which looks like:</p>
  
  <pre><code>+-----+-----+-----+
  |     | l1  | l2  |
  +-----+-----+-----+
  |  0  | A   | 1   |
  +-----+-----+-----+
  |  1  | A   | 2   |
  +-----+-----+-----+
  |  2  | B   | 1   |
  +-----+-----+-----+
  |  3  | B   | 2   |
  +-----+-----+-----+
  </code></pre>
  
  <p>Note, the first column is the index.</p>