title: Numpy union arrays in order
body: <p>I've got three different numpy arrays</p>
  
  <pre><code>a = array([ 0,  3,  6,  9, 12])
  b = array([ 1,  4,  7, 10, 13])
  c = array([ 2,  5,  8, 11, 14])
  </code></pre>
  
  <p>How can I join them using numpy methods that </p>
  
  <pre><code>d = array[(0,1,2,3,4,...,12,13,14)]
  </code></pre>
  
  <p>I don't want to write a loop like</p>
  
  <pre><code>for i in range(len(a)):
   [...]
  </code></pre>
  
  <p>This is only an example in my project the arrays are not sorted and I want to keep their order. </p>