title: How to transpose an RDD in Spark
body: <p>I have an RDD like this:</p>
  
  <pre><code>1 2 3
  4 5 6
  7 8 9
  </code></pre>
  
  <p>It is a matrix. Now I want to transpose the RDD like this:</p>
  
  <pre><code>1 4 7
  2 5 8
  3 6 9
  </code></pre>
  
  <p>How can I do this?</p>