HybridTrajectoryOptimization and minor updates

Added HybridTrajectoryOptimization and Compass Gait example. Minor changes to ContactImplicitTrajectoryOptimization, particularly the initialization of slack variables.
No, it's not the same. This is in `NonlinearComplementarityConstraint` which is a general purpose class, mode is the option choice for how to model the NC constraint.  There are other ways to treat these (that do use slack variables) that I haven't included, but could be added later.