title: Converting space separated integers to lists in a dataframe
body: <p>I have a dataframe that looks like this :</p>
  
  <pre><code>[['label1', 'label2']
    ['1 2 3', '1 2 3']
    ['4 5 6', '4 5 7']]
  </code></pre>
  
  <p>So each column is considered as strings. I would like to apply a l2 distance between each element of the two labels so I need to convert the columns to list of float.</p>
  
  <p>Is there a "clean" way to do that, better than a double for loop with something like <code>[float(x) for x in element.split()]</code> ?
  (My dataframe is huge so I would like something quite optimized)</p>