title: How to extend numpy arrray
body: <p>I have 2 numpy arrays and i want to combine these two array together using extend. 
  eg:</p>
  
  <pre><code>a = [[1,2,3],[4,5,6],[7,8,9]]
  b = [[0,0,0],[1,1,1]]
  </code></pre>
  
  <p>what i want is
  <code>c = [[1,2,3],[4,5,6],[7,8,9],[0,0,0],[1,1,1]]</code></p>
  
  <p>It seems that I cannot use <code>extend</code> as python list. otherwise it will raise <code>AttributeError: 'numpy.ndarray' object has no attribute 'extend'</code> error. </p>
  
  <p>Currently I tried by transforming them into lists :</p>
  
  <pre><code>a_list = a.tolist()
  b_list = b.tolist()
  a_list.extend(b_list)
  c = numpy.array(a_list)
  </code></pre>
  
  <p>I wonder if any better solution exist? </p>