Physic: all bricks are implemented
 * PhysicBrick is no more an abstract class, and its children have been
deleted.
 * PhysicBrick instantiate itself with the right masks according to the
BrickType of the Brick given as an argument to its constructor.
 * Brick's masks are stored in text files in the "data" folder.
 * A Brick's mask file's name has to be of the form "BrickX.mask" where
'X' is the value of the BrickType in the enum BrickType (e.g: 'I',
'O'...)