Client : add sum of mass and gravity center into QuadTree class

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>
Client : fix a bug into the method calculatCenterOfGravity

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>