Fix a bug in the integrator stepBy() method that prevented it from working when the previous state was interpolated (the normal case).
Fix a bug in the integrator stepBy() method that prevented it from working when the previous state was interpolated (the normal case).

Gjacquenot/simbody,simbody/simbody,scamille/simbody,Gjacquenot/simbody,scamille/simbody,elen4/simbody,Gjacquenot/simbody,elen4/simbody,scamille/simbody,robojukie/simbody,robojukie/simbody,chrisdembia/simbody,chrisdembia/simbody,elkingtonmcb/simbody,elkingtonmcb/simbody,elkingtonmcb/simbody,robojukie/simbody,chrisdembia/simbody,simbody/simbody