title: Python argument parser list of list or tuple of tuples
body: <p>I'm trying to use argument parser to parse a 3D coordinate so I can use </p>
  
  <pre><code>--cord 1,2,3 2,4,6 3,6,9
  </code></pre>
  
  <p>and get </p>
  
  <pre><code>((1,2,3),(2,4,6),(3,6,9))
  </code></pre>
  
  <p>My attempt is</p>
  
  <pre><code>import argparse
  parser = argparse.ArgumentParser()
  parser.add_argument('--cord', help="Coordinate", dest="cord", type=tuple, nargs=3)
  args = parser.parse_args(["--cord","1,2,3","2,4,6","3,6,9"])
  
  vars(args) 
  {'cord': [('1', ',', '2', ',', '3'),
    ('2', ',', '4', ',', '6'),
    ('3', ',', '6', ',', '9')]}
  </code></pre>
  
  <p>What would the replacement of the comma be?</p>