Reimplement PositionDotDot method in PointInPlaneWithStiction constraint for better performance and to match VelocityDot implementation.
Minor fixes in doxygen comments.
Added new Constraint BallSlidingOnPlane. This is a warm-up for BallRollingOnPlane which is needed for unilateral contact between a ball and plane.

There are some changes to BallRollingOnPlane here but they should be ignored; it is non-functional.
Minor change to BallSlidingOnPlane API: return MobilizedBody references rather than MobilizedBodyIndex values which are inconvenient to work with.