title: How to concatenate 2 columns,which contains np arrays, into a new column in dataframe
body: <p>original dataframe</p>
  
  <pre><code>A                      B
  np.array([1, 2, 3])    np.array([4, 5, 6])
  np.array([7, 8, 9])    np.array([9, 10, 11])
  </code></pre>
  
  <p>wish it to be </p>
  
  <pre><code>A                      B                         C
  np.array([1, 2, 3])    np.array([4, 5, 6])       np.array([1, 2, 3, 4, 5, 6])
  np.array([7, 8, 9])    np.array([9, 10, 11])     np.array([7, 8, 9, 9, 10, 11])
  </code></pre>
  
  <p>How to achieve that?</p>