Client : fix a bug into the method to calculate the gravity center

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>
Reviewed-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
Client : integration of the Barnes-Hut Algorithm

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>
Client : apply modification after review by Sébastien

Signed-off-by: Jean-François Erdelyi <jean-francois.erdelyi@etu.iut-tlse3.fr>
Reviewed-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
use a copy of points in the calculation of gravity center

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>