title: Get row numbers of rows matching a condition in numpy
body: <p>Suppose I have a numpy array like:</p>
  
  <pre><code>a = array([[1, 2, 3],
         [4, 5, 6],
         [7, 8, 9],
         [3, 2, 1]])
  </code></pre>
  
  <p>I want to check if the second element == 2. </p>
  
  <p>I know I can do this:</p>
  
  <pre><code>&gt;&gt;&gt; a[:,1]==2
  array([ True, False, False,  True], dtype=bool)
  </code></pre>
  
  <p>returning booleans. My question is, how do I get the row numbers of the rows where the condition is true? In this example I would want to get back <code>array([0, 3])</code> because the 0th and 3rd rows match the condition second element == 2. </p>