fix signed/unsigned warning
fix signed/unsigned warning

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