fix in stiffness matrix computation
fix in stiffness matrix computation

Milad-Rakhsha/chrono,armanpazouki/chrono,rserban/chrono,rserban/chrono,dariomangoni/chrono,jcmadsen/chrono,Bryan-Peterson/chrono,dariomangoni/chrono,projectchrono/chrono,hsu/chrono,Bryan-Peterson/chrono,jcmadsen/chrono,tjolsen/chrono,jcmadsen/chrono,rserban/chrono,andrewseidl/chrono,dariomangoni/chrono,armanpazouki/chrono,jcmadsen/chrono,projectchrono/chrono,hsu/chrono,dariomangoni/chrono,projectchrono/chrono,tjolsen/chrono,armanpazouki/chrono,PedroTrujilloV/chrono,amelmquist/chrono,Bryan-Peterson/chrono,tjolsen/chrono,Bryan-Peterson/chrono,andrewseidl/chrono,rserban/chrono,jcmadsen/chrono,projectchrono/chrono,projectchrono/chrono,amelmquist/chrono,hsu/chrono,armanpazouki/chrono,andrewseidl/chrono,rserban/chrono,PedroTrujilloV/chrono,rserban/chrono,PedroTrujilloV/chrono,dariomangoni/chrono,hsu/chrono,Milad-Rakhsha/chrono,andrewseidl/chrono,tjolsen/chrono,amelmquist/chrono,Milad-Rakhsha/chrono,andrewseidl/chrono,PedroTrujilloV/chrono,Milad-Rakhsha/chrono,hsu/chrono,PedroTrujilloV/chrono,Bryan-Peterson/chrono,amelmquist/chrono,Milad-Rakhsha/chrono,jcmadsen/chrono,amelmquist/chrono,amelmquist/chrono,rserban/chrono,armanpazouki/chrono,jcmadsen/chrono,tjolsen/chrono,dariomangoni/chrono,armanpazouki/chrono,Milad-Rakhsha/chrono,projectchrono/chrono