Ready to merge: Fix information leakage between polyhedra interactions that was caused by the 'shearForce' declaration in the 'Law2_PolyhedraGeom_PolyhedraPhys_Volumetric' class.

As pointed out in (answers#4142) and (answers#4102). Polyhedra sliding simulations gave unrealistic results. Also, adding another body to the simulation changed the behaviour of the others (not even in contact). The reason was the global 'shearForce' declaration in the volumetric Law2. Based on this shear force from one interaction was a start point for computing another one. This declaration explicitly said that it is for debugging interactions between two bodies. Unfortunately, it broke the behaviour when more were included. I recommend removing this declaration altogether (to keep the names in the rest of the code consistent). The proposed change fixed both the mentioned issues. Now, I prepared a draft MR to quickly change if id didn't break anything. If so, I think it could be quickly merged, since the changes are minimal.

Cheers, Karol