Skeleton has mInitPose and SkeletonDynamics has mInitQDotVector. SkeletonDynamics is able to backup its state with backupInitState() and restore with restoreInitState().

World is able to reset the states of all skeletons with reset(().

TODO: We can consider better ways for this functionalities.
Merge remote-tracking branch 'refs/remotes/origin/master' into state_backup
Merge branch 'refs/heads/state_backup' into gazebo_support