title: PySpark reduceByKey on multiple values
body: <p>If I have a K,V pair that is like:</p>
  
  <pre><code>(K, (v1, v2))
  (K, (v3, v4))
  </code></pre>
  
  <p>How can I sum up the values such that I get
  <code>(k, (v1 + v3, v2 + v4))</code> ?</p>