added a Differentiator example
added a Differentiator example

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