title: divide the list into rows (create a nested list from a list)
body: <p>this probably has a super-easy solution, but I cannot get it right.</p>
  
  <p>I have a list of 400 integers. Now. I want to divide this list so that the first 20 go in one list, the second 20 in another, etc. In the end I want to have a 20x20 nested list.</p>
  
  <p>Any ideas? Thanks!</p>