vc
Assume real COR=1 when v=0.
e_min = given minimum COR, at v >= vp (a.k.a. plastic_coef_restitution)
d = slope = (1-e_min)/vp
OR, e_min = 1 - d*vp
e_max = maximum COR = 1-d*vc, reached at v=vc
e = 0, v <= vc
= 1 - d*v, vc < v < vp
= e_min, v >= vp
dissipation factor = d*min(v,vp) [compliant]
cor = e [rigid]
Combining rule e = 0, e1==e2==0
= 2*e1*e2/(e1+e2), otherwise]]>