title: Multi-dimensional ndarray set operations
body: <p>I have the need to run the set difference of two potentially large multidimensional <code>numpy.ndarrays</code>.<br>
  My tests have proven unsuccessful trying to extend the <code>numpy.setdiff1d</code> to work for multidimensional arrays without throwing the following <code>TypeError</code>:</p>
  
  <pre><code>TypeError: Cannot change data-type for object array.
  </code></pre>
  
  <p>The data in these arrays is a long and a string.</p>
  
  <p>I have the two following arrays:</p>
  
  <pre><code>A = [[0L 'a']
       [2L 'c']
       [1L 'b']
       [3L 'b']
       [0L 'd']]
  
  B = [[1L 'b']
       [0L 'a']
       [4L 'b']]
  </code></pre>
  
  <p>The expected result is the set difference <code>S = A - B</code> which are the pairs <code>((0,d), (2,c), (3,b))</code>.</p>