Refactored so that solvers now mainly work with a target location in the form of a Pair<Int, Int> object as opposed to a rowKey and a columnKey passed seperately.
Cleaned Sudoku a little: made braces uniform, made sure the overloaded get() called the other, and removed unused imports.
Renames cellCanBeN() to directConflictExistsBetween() and moves it to the Sudokus utility class.
Draft of OnlyPossibleNumberSolver which, given some shared code with SimpleEliminationSolver, turned out to be pretty short.