title: subsampling every nth entry in a numpy array
body: <p>I am a beginner with numpy, and I am trying to extract some data from a long numpy array. What I need to do is start from a defined position in my array, and then subsample every nth data point from that position, until the end of my array. </p>
  
  <p>basically if I had</p>
  
  <pre><code>a = [1,2,3,4,1,2,3,4,1,2,3,4....] 
  </code></pre>
  
  <p>I want to subsample this to start at <code>a[1]</code> and then sample every fourth point from there, to produce something like</p>
  
  <pre><code>b = [2,2,2.....]
  </code></pre>