title: Summing positive and negative elements from two NumPy arrays
body: <pre><code>&gt;&gt;&gt; x1
  array([[ 0.,  -1.,   2.],
         [ 3.,  -4.,  2.],
         [ -2.,  1.,  -8.]])
  
  &gt;&gt;&gt; x3
  array([[ 0.,  -5.,  2.],
         [ 3.,  0.,  -3.],
         [ 3.,  2.,  8.]])
  </code></pre>
  
  <p>I need two matricies to be output: <code>S</code> and <code>T</code>, such that <code>X</code> is the sum of all positive values in <code>X</code> and <code>Y</code>, and <code>T</code> is the sum of all negative values in <code>X</code> and <code>Y</code>.</p>
  
  <p>For example:</p>
  
  <pre><code>      S = array([   [ 0.,  0.,  4.],
                      [ 6.,  0.,  2.],
                      [ 3.,  3.,  8.]])
  
        T = array([   [ 0.,  -6.,  0.],
                      [ 0.,  -4.,  -3.],
                      [ -2.,  0.,  -8.]])  
  </code></pre>
  
  <p>I am using Python 2.6.7.</p>