title: Spark Scala Dataframe How to create new column with two or more existing columns
body: <p>Suppose I have a Spark Scala DataFrame object like:</p>
  
  <p><code>
  +----------------+
  |col1  |col2     |
  +----------------+
  |1     |2        |
  |3     |4        |
  +----------------+
  </code></p>
  
  <p>And I want a DataFrame like:
  <code>
  +--------------------+
  |col1  |col2  |col3  |
  +--------------------+
  |1     |2     |3     |
  |3     |4     |7     |
  +--------------------+
  </code></p>
  
  <p>Which adds <code>col1</code> and <code>col2</code> to <code>col3</code>, could anyone please tell me how to do that? <code>WithColumn</code> takes only one column as parameter whereas I need to take two columns.</p>