Expose viscous shear force in ViscElPhys

The viscous part of the tangential contact force (shearForceVisc = cs*shearVelocity) is now stored in ViscElPhys as well as accessible from Python. It is zero in the slip regime, i.e. when the Mohr-Coulomb criterion is violated, since in that case only the elastic shear force, this latter capped at the friction limit, acts on the contact.