title: numpy: concatenate two arrays along the 3rd dimension
body: <p>I would like to add another "slice" of data to an existing data cube:</p>
  
  <pre><code>import numpy as np
  
  a = np.array([[[1, 1, 1, 1], [2, 2, 2, 2], [3, 3, 3, 3]], [[4, 4, 4, 4], [5, 5, 5, 5], [6, 6, 6, 6]]])
  print(a.shape)  # (2, 3, 4)
  
  b = np.array([[7, 7, 7], [8, 8, 8]])
  print(b.shape)  # (2, 3)
  
  c = np.concatenate([a, b], axis=2)  # ValueError: all the input arrays must have same number of dimensions 
  print(c.shape)  # wanted result: (2, 3, 5)
  </code></pre>
  
  <p>So I basically want to add the 2x3 array to the 2x3x4 array by extending the last dimension to 2x3x5. The value error does not quite make sense to me as the array can't be the same shape? What am I doing wrong here?</p>