title: How to append different sizes of numpy arrays and put na's in empty location
body: <p>I want to add a new row to a numpy 2d-array, say if array 1 has dimensions of (2, 5) and array-2 is a kind of row (which has 3 values or cols) of shape (3,)
  my resultant array should look like (3, 10) and the last two indices in 3rd row should be NA's.</p>
  
  <pre><code>arr1 = array([[9, 4, 2, 6, 7],
                [8, 5, 4, 1, 3]])
  
  arr2 = array([3, 1, 5])
  
  after some join or concat operation
  
  arr1_arr2 = array([[9, 4, 2, 6, 7],
                     [8, 5, 4, 1, 3], 
                     [3, 1, 5, np.nan, np.nan]])
  </code></pre>
  
  <p>I have tried numpy append concat functions but they dont work this way. </p>