client: fix the variable name for the object center

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
Client : change name of variable overlap

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>
Client : Replace the grid by QuadTree

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