Fix doxygen comments for Force::Gravity. Fixes issue #80.
Fix doxygen comments for Force::Gravity. Fixes issue #80.

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