title: How to explode columns?
body: <p>After:</p>
  
  <pre><code>val df = Seq((1, Vector(2, 3, 4)), (1, Vector(2, 3, 4))).toDF("Col1", "Col2")
  </code></pre>
  
  <p>I have this DataFrame in Apache Spark: </p>
  
  <pre><code>+------+---------+
  | Col1 | Col2    |
  +------+---------+
  |  1   |[2, 3, 4]|
  |  1   |[2, 3, 4]|
  +------+---------+
  </code></pre>
  
  <p>How do I convert this into:</p>
  
  <pre><code>+------+------+------+------+
  | Col1 | Col2 | Col3 | Col4 |
  +------+------+------+------+
  |  1   |  2   |  3   |  4   |
  |  1   |  2   |  3   |  4   |
  +------+------+------+------+
  </code></pre>