title: How to get date from different year, month and day columns in spark (scala)
body: <p>I have a DataFrame including data like:</p>
  <pre><code>+----+-----+---+-----+
  |Year|Month|Day|...  |
  +----+-----+---+-----+
  |2012|    2| 20|     |
  |2011|    7|  6|     |
  |2015|    3| 15|     |
  </code></pre>
  <p>and I would like to add a column with date</p>