Fix bug in level-2 normal addition.
To reproduce the error, take a point X and compare the results from the add and diff_add with P=Q=X.
[A venir] Mise en place de quotas sur nos instances GitLab. Plus d'information sur : https://doc-si.inria.fr/pages/viewpage.action?pageId=129728565
[Coming soon] Quotas installation on our GitLab instances. More information: https://doc-si.inria.fr/pages/viewpage.action?pageId=129728565
To reproduce the error, take a point X and compare the results from the add and diff_add with P=Q=X.