Restore computing constraint force vector: Skeleton::getConstraintForceVector()
Fix no return in function returning non-void
Uncomment back unintended commented code
Rename setConstraintImpulses() -> setJointConstraintImpulses()
- and getConstraintImpulses() -> getJointConstraintImpulses()
- to make clear the meanings
Merge pull request #222 from dartsim/constraint_impulse_in_gen_coord

Restore Skeleton::getConstraintForceVector()