eloMatrix property Null safety
read / write
        The elo matrix. This matrix is used to calculate the elo gained by a user.
It has the following structure:
Easy  Med  Hard Extr
   1, 1.5, 2.5,  5, Fast
  1.4, 1.9, 2.9,  6, Medium
    2, 2.5, 3.5,  8, Large
  2.8, 3.3, 4.3, 10, Marathon
The calculation is performed by taking the row (duration) and dot-multiplying it with the column (difficulty of the question).
The values have been chosen that way to encourage users to play longer and harder games.
Implementation
static List<List<double>> eloMatrix =
[
  [   1, 1.5, 2.5,  5],
  [ 1.4, 1.9, 2.9,  6],
  [   2, 2.5, 3.5,  8],
  [ 2.8, 3.3, 4.3, 10],
];