title: How to flatten list inside RDD?
body: <p>Is it possible to flatten list inside RDD? For example convert:</p>
  
  <pre><code> val xxx: org.apache.spark.rdd.RDD[List[Foo]]
  </code></pre>
  
  <p>to:</p>
  
  <pre><code> val yyy: org.apache.spark.rdd.RDD[Foo]
  </code></pre>
  
  <p>How to do this?</p>