title: Efficient numpy indexing: Take first N rows of every block of M rows
body: <pre><code>x = np.array([1,2,3,4,5,6,7,8,9,10,11,12,13,14,15])
  </code></pre>
  
  <p>I want to grab first 2 rows of array x from every block of 5, result should be:</p>
  
  <pre><code>x[fancy_indexing] = [1,2, 6,7, 11,12]
  </code></pre>
  
  <p>It's easy enough to build up an index like that using a for loop.</p>
  
  <p>Is there a one-liner slicing trick that will pull it off? Points for simplicity here.</p>