title: numpy: Counting in a 2D array where an element satisfies a condition
body: <p>I have a numpy array like below. I need a count of rows where the first element is 2. So in the array below, four rows start with 2 - the answer would be 4. How is this best accomplished in numpy? (I cannot use pandas, but can use scipy).</p>
  
  <pre><code>array([[1, 4, 5],
         [1, 4, 5],
         [2, 4, 5],
         [2, 4, 5],
         [2, 4, 5],
         [2, 4, 5],
         [3, 4, 5],
         [3, 4, 5],
         [3, 4, 5],
         [3, 4, 5],
         [3, 4, 5],
         [3, 4, 5]])
  </code></pre>