Add utility class for specifying a linear spring-damper element.
Add utility class for specifying a linear spring-damper element.

This can be used as the callback function for a ChLinkSpringCB element.

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