Now applying the Newton's 3rd Law pair to Ground.
Now applying the Newton's 3rd Law pair to Ground.

Prior to this commit, the force generated by ExponentialSpringForce was applied only to the MobilizedBody, which is a clear violation of Newton's 3rd Law:

"If Body A applies a force to Body B, then Body B applies a force to Body A that is equal in magnitude and opposite in direction."

With this commit, the equal and opposite force is now properly also applied to Ground.  This should generate a reaction force registered by Ground, which provides the basis for another test that should be added to TestExponentialSpring.cpp.

I also got rid of an unused member variable called useBlended.

simbody/simbody,simbody/simbody