title: How to inserts rows in a numpy array based on the condition
body: <p>I have two lists of numpy arrays and want to insert arrays of a list into arrays of another list:</p>
  <pre><code>big_list=[np.array([[1., 1., 2.], [1., 1., 1.9], [1., 1., 5.2]]),
            np.array([[1., 3., 5.1], [0., 0., 1.2], [1., 1., 1.4]])]
  
  small_list= [np.array([[-1., -1., -5.]]),
               np.array([[-1., -1., -1.], [0., -2., -0.5]])]
  </code></pre>
  <p>I want to insert first array of <code>small_list</code> in first array of <code>big_list</code> and also second array of <code>small_list</code> in second array of <code>big_list</code>. I want to put <code>small_list</code> where the third column of <code>big_list</code> has a notable change. In first array of <code>big_list</code>, from second row to third row, the thrid column changes a lot (from <code>1.9</code> to <code>5.2</code>). For the second array of <code>big_list</code>, the changes occurs after the first row (from <code>5.1</code> to <code>1.2</code>). For example I want to use a threshold like <code>3</code>, for the difference of the third column of a row with the next row. So, I want  to have:</p>
  <pre><code>merged= [np.array([[1., 1., 2.], [1., 1., 1.9], [-1., -1., -5.], [1., 1., 5.2]]),
           np.array([[1., 3., 5.1], [-1., -1., -1.], [0., -2., -0.5], [0., 0., 1.2], [1., 1., 1.4]])]
  </code></pre>
  <p>At the moment, my code can only append rows of <code>small_list</code> at the end of <code>big_list</code>:</p>
  <pre><code>merged = [np.append(array, to_append, axis=0) for (array, to_append) in zip(big_list, small_list)]
  </code></pre>
  <p>Is there a way to insert thes rows where I want?
  In advance, I do appreciate any help.</p>