title: create 3D binary image
body: <p>I have a 2D array, <code>a</code>, comprising a set of 100 x,y,z coordinates: </p>
  
  <pre><code>[[ 0.81  0.23  0.52]
   [ 0.63  0.45  0.13]
   ...
   [ 0.51  0.41  0.65]]
  </code></pre>
  
  <p>I would like to create a 3D binary image, <code>b</code>,  with 101 pixels in each of the x,y,z dimensions, of coordinates ranging between 0.00 and 1.00. 
  Pixels at locations defined by <code>a</code> should take on a value of 1, all other pixels should have a value of 0.</p>
  
  <p>I can create an array of zeros of the right shape with <code>b = np.zeros((101,101,101))</code>, but how do I assign coordinate and slice into it to create the ones using <code>a</code>?</p>