title: Count number of nan values on a matrix with strings
body: <p>I want to count the number of 'nan' values per column inside a matrix full of string values. Like this one:
  m:</p>
  <pre><code>    [['CB_2' 'CB_3']
   ['CB_1-1' 'CB_4-1']
   ['CB_1-2' 'CB_4-2']
   ['CB_2-1' 'CB_5-1']
   ['CB_2-2' 'CB_5-2']
   [nan 'CB_6-1']
   [nan 'CB_6-2']]
  </code></pre>
  <p>I tried using <code>np.count_nonzero(~np.isnan(m)</code> but it seems to work only with numerical values. Perhaps if I convert it into an empty string or zero (?).</p>
  <p>Also, I created a sample numpy array with strings (to try several options) <code>(np.array([['a','b'],['c','d'],['e','f'],['e','g'],['k','ñ'],['w','q'],['y','d']]))</code> but when I use <code>np.nan</code> it doesnt seems to works correctly since it adds the <code>nan</code> value as a string (<code>'nan'</code>).</p>
  <p>Thanks,</p>