title: PySpark: How to write a Spark dataframe having a column with type SparseVector into CSV file?
body: <p>I have a spark dataframe which has one column with type spark.mllib.linalg.SparseVector:</p>
  
  <p>1) how can I write it into a csv file?</p>
  
  <p>2) how can I print all the vectors?</p>