Fix 2 bugs linked to the Grids :
- in utils.py : remove a factor 2 in the calculation of the bending stiffness kr.
- in Grid.cpp : fix a bug that occured in the Sphere-GridConnection contact. The contact was not detected in a very specific case : if the sphere was next to a node and the node goes from a convex (angle>180°) form to a concav (angle<180°) form.