title: Replicate rows in a Pyspark Dataframe
body: <p>I have the following dataframe:</p>
  
  <pre><code>id, test, date
  1, A, 01/20/2020
  1, B, 01/25/2020
  2, A, 02/20/2020
  2, B, 02/25/2020
  2, C, 02/25/2020
  </code></pre>
  
  <p>Since the number of distinct tests is 3(A,B,C) I want a row for test C to be inserted for id 1 with date as "NA"</p>
  
  <p>The resulting dataframe should be:</p>
  
  <pre><code>id, test, date
  1, A, 01/20/2020
  1, B, 01/25/2020
  1, C, NA
  2, A, 02/20/2020
  2, B, 02/25/2020
  2, C, 02/25/2020
  </code></pre>