title: count unique combinations of values
body: <p>My dataframe looks like this:</p>
  
  <pre><code>ID | value 1 | value 2 | value 3 | value 4
  1  |    M    |    D    |    F    |   A
  2  |    F    |    M    |    G    |   B
  3  |    M    |    D    |    F    |   A
  4  |    L    |    D    |    E    |   B
  </code></pre>
  
  <p>I want to get something like this.</p>
  
  <pre><code>value 1 | value 2 | value 3 | value 4|  Number of combinations
    M     |    D    |    F    |   A    |     2
    F     |    M    |    G    |   B    |     1
    L     |    D    |    E    |   B    |     1
  </code></pre>
  
  <p>e.g. to count the number of unique combinations of the columns value 1 - value 4.</p>