title: Spark: unpersist RDDs for which I have lost the reference
body: <p>How can I unpersist RDD that were generated in an MLlib model for which I don't have a reference? </p>
  
  <p>I know in pyspark you could unpersist all dataframes with <code>sqlContext.clearCache()</code>, is there something similar but for RDDs in the scala API? Furthermore, is there a way I could unpersist only some RDDs without having to unpersist all? </p>