title: Add metadata comment to Numpy ndarray
body: <p>I have a Numpy ndarray of three large arrays and I'd just like to store the path to the file that generated the data in there somewhere. Some toy data:</p>
  
  <pre><code>A = array([[  6.52479351e-01,   6.54686928e-01,   6.56884432e-01, ...,
                2.55901861e+00,   2.56199503e+00,   2.56498647e+00],
             [             nan,              nan,   9.37914686e-17, ...,
                1.01366425e-16,   3.20371075e-16,  -6.33655223e-17],
             [             nan,              nan,   8.52057308e-17, ...,
                4.26943463e-16,   1.51422386e-16,   1.55097437e-16]],                 
             dtype=float32)
  </code></pre>
  
  <p>I can't just append it as an array to the ndarray because it needs to be the same length as the other three. </p>
  
  <p>I could just append <code>np.zeros(len(A[0]))</code> and make the first value the string so that I can retrieve it with A[-1][0] but that seems ridiculous.</p>
  
  <p>Is there some metadata key I can use to store a string like <code>/Documents/Data/foobar.txt'</code> so I can retrieve it with something like <code>A.metadata.comment</code>?</p>
  
  <p>Thanks!</p>