title: Create indicator matrix from two arrays in Python Numpy
body: <p>Given two vectors, I would like to create an indicator matrix. For example, given <code>a=np.array([5,5,3,4,4,4])</code>, and <code>b=np.array([5,4,3])</code>, the result should be</p>
  
  <pre><code>   5 4 3
  
  5  1 0 0
  5  1 0 0
  3  0 0 1
  4  0 1 0
  4  0 1 0
  4  0 1 0
  </code></pre>
  
  <p>What is the simplest way to achieve this?</p>