- Added ConstantCoordinate Constraint (a position-level mobility lock).
- Added HardStopLower and HardStopUpper unilateral contact constraints that use ConstantCoordinate as their underlying Constraint.
- Modified RigidContact1 to add stops to the Pencil example and left it set to run that way.
- Implemented unilateral contact sign convention (needed since upper/lower stops have opposite conventions).
- Fixed equation documentation in the PLUS solver.
- Modified Constraint so that it can be marked "conditional", for use by conditional constraint-aware solvers (not currently in use).
- Add missing virtual destructor to ImpulseSolver class (thanks, @chrisdembia).