Removed the SpringZeroRecorder from ExponentialSpringForceImpl
Removed the SpringZeroRecorder from ExponentialSpringForceImpl

At one point, I tried recording the spring zero using a PeriodicEventReporter so that I could get an average spring zero velocity.  However, average spring zero velocity turned out not to be a very useful for transitioning the Sliding state.

simbody/simbody,simbody/simbody