mutable and immutable cells

app/src/main/java/sudoku/SudokuFactory.java

@@ -8,7 +8,7 @@ public class SudokuFactory {
 	private static List<Cell> initCells(int size) {
 		List<Cell> cells = new ArrayList<>(size * size);
 		for (int i = 0; i < size * size; i++) {
-			cells.add(new Cell());
+			cells.add(new MutableCell());
 		}
 		return cells;
 	}