title: Group by elements in list
body: <p>I am newbie in PySpark.
  I created spark dataframe,and I have a column "Countries" which contains list of countries.
  How can i groupBy my dataframe by single countries which exists in Country list</p>
  
  <pre><code>+-----------------+
  |        countries|
  +-----------------+
  |  [Россия, Китай]|
  | [Великобритания]|
  |       [Норвегия]|
  |         [Россия]|
  |               []|
  |            [США]|
  |         [Россия]|
  |            [США]|
  |               []|
  |         [Россия]|
  |               []|
  |               []|
  |         [Италия]|
  | [Россия, Грузия]|
  |            [США]|
  |               []|
  |               []|
  |               []|
  |[Великобритания ]|
  |       [Беларусь]|
  +-----------------+
  </code></pre>