BoardLines refactoring
Error handling when input is an invalid cell
Cosmetic changes
isEmpty(cell) renamed to isEmptyAt(cell)
A few methods renamed on Marks
isEmpty method on Marks removed. It was not used
containsSingleMark() method deleted from Marks. It was unused
hasAnyCornerMarked() method removed from Marks. It was unused
oppositeCornerOf(cell) on Marks changed to private
emptyOppositeCell(cell) method on Marks removed. It was not used
cornerMarksFor(playerMark) on Marks removed. Not being used