Add drakeFunction.interpolation.Linear class
Fixes to SDFKP
Fix spelling of concatenate in drakeFunction.Affine
Cleanup LeggedRobotPlanningProblem

Also adds overloaded addConstraint methods for CDFKP and KinematicDirtran
that allow RigidBodyConstraints to be added via addConstraint.
Add brief documentation for KinematicDirtran class.