title: Subset numpy array based on another array
body: <p>I have 2 numpy arrays:</p>
  
  <pre><code>arr_a = array(['1m_nd', '2m_nd', '1m_4wk'],
        dtype='&lt;U15')
  
  arr_b = array([0, 1, 1])
  </code></pre>
  
  <p>I want to select elements from <code>arr_a</code> based on <code>arr_b</code>. I am doing this:
  <code>arr_a[arr_b]</code>, but I get this as result: </p>
  
  <pre><code>array(['1m_nd', '2m_nd', '2m_nd'],
        dtype='&lt;U15')
  </code></pre>
  
  <p>instead of:</p>
  
  <pre><code>array(['2m_nd', '1m_4wk'],
        dtype='&lt;U15')
  </code></pre>
  
  <p>How do i fix this?</p>