Made difficulty modes to public such that others can use them without
the need of recreating them in their own classes.
Made 2 lists of integers to keep track of the numbers that have tried
and numbers that are available for backtracking.
Modified code to use the tracking lists of numbers. Still haven't
implement backtracking.
Have backtracking working. An IllegalArgumentException occurs most of
the time. Sometimes it get passed and produce the sudoku.