title: Insert an array into another array in numpy?
body: <p>How do I do this? (in a computationally efficient way)</p>
  <pre><code>arr1 = np.array([0, 4, 8, 12, 16])
  arr2 = np.array([1, 5, 9, 13, 17])
  arr3 = np.array([2, 6, 10, 14, 18])
  arr4 = np.array([3, 7, 11, 15, 19])
  
  what_i_want = [ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19]
  </code></pre>