Change in the calculation of the normal and tangential stiffness and damping. The change affects only the behavior
when one of the two parameters is zero, e.g. for two particles with different stiffness k1 and k2, the contact
stiffness will always be k = k1*k2/(k1+k2). Before it gave the same except when k2 = 0 (respectively k1 = 0), where it
gave k = k1 (resp. k=k2). This is done to ensure continuity in the behavior when one of the two parameter tend
to zero.
Add a function contactParameterCalculation in Ip2_ViscElMat_ViscElMat_ViscElPhys to avoid code duplication.