title: Flatten array of arrays column without UDF's
body: <p>Is there a way to <em>flatten</em> a column which contains array of arrays without using UDF's (in case of DataFrames)?</p>
  
  <p>For example:</p>
  
  <pre><code>+---------------------------------------------------------------------------------------------------------+
  |vector                                                                                                   |
  +---------------------------------------------------------------------------------------------------------+
  |[[106.0,1006.0,26.0], [107.0,1007.0,27.0], [108.0,1008.0,28.0]]                                          |
  |[[100.0,1000.0,20.0]]                                                                                    |
  |[[101.0,1001.0,21.0], [102.0,1002.0,22.0], [103.0,1003.0,23.0], [104.0,1004.0,24.0], [105.0,1005.0,25.0]]|
  +---------------------------------------------------------------------------------------------------------+
  </code></pre>
  
  <p>should be converted to</p>
  
  <pre><code>+---------------------------------------------------------------------------------------------------------+
  |vector                                                                                                   |
  +---------------------------------------------------------------------------------------------------------+
  |[106.0,1006.0,26.0,107.0,1007.0,27.0,108.0,1008.0,28.0]                                         
  |[100.0,1000.0,20.0]                                                                                   
  |[101.0,1001.0,21.0,102.0,1002.0,22.0,103.0,1003.0,23.0,104.0,1004.0,24.0,105.0,1005.0,25.0]|
  +---------------------------------------------------------------------------------------------------------+
  </code></pre>