can move down routine implemented and its tests
out routine at CELL_2048 and its respective test implemented
Changes in BOARD_2048

Change implemenation of make_empty to fix a warning and change the name
of features rows to Rows and columns to Colums (this are constants, the
name should begin with uppercase)
Fix warnings of classs CONTROLLER_2048

fix warning in class CONTROLLER_2048
implement nr_of_filled_cells in BOARD_2048
.
Changes in postcondition and Board tests