Give names to SimTK::Constraints.

[chrisdembia]

Right now, Moco writes the values of Lagrange multipliers to Storage files. The names used for these multipliers multiplier variables are sensitive to the order in which constraints appear in a model. This is because we loop through constraints at the Simbody level, not the OpenSim level.

Unfortunately, there is no current way to go from a SimTK::Constraint back to the OpenSim component that added the constraint. OpenSim should give names to SimTK::Constraints and pass those on to Simbody. This requires adding support in Simbody for constraints to have names. Then, OpenSim can provide meaningful names that we can use in Moco.