[Bug] Magnetic grid steps not taken into account

User story : As a user, I want the magnetic grid steps defined in the geometrical preferences of the application to be taken into account, in order to ease the modelisation.

By the way, allow step value of 0.5 in order to be able to draw symetrical line with odd length.

Analysis : Currently, the step is hard-coded at one in the code in file TYAbstractSceneEditor.cpp :
/// Pas de la grille imaginaire de positionnement
static const double SnapGridStep = 1.0;