GameBoard : board is no more represented by a Cell[][] but by a
Brick[][] instead
 * deletion of Cell inner class
 * PhysicSolver uses the Brick class directly and access its physical
component when needed
Brick class can no more flip itself. Flipping can be done only with
PhysicBrick class.
 * To flip the current brick you must use the PysicSolver. If you do
not, you don't know 
   if the brick could actually flip in the board without colliding.
 * The PhysicBrick should not be accessible through the Brick class but
decorate the Brick class
   instead. TO DO: See if the decorator thing can be done in the current
architecture.