Add default value of n in drakeFunction.Difference
Fix errors in drakeFunction.Difference
Add sparsity to basic DrakeFunctions.

Implemented for:
  * DrakeFunctions (default = full)
  * Composed
  * Affine
  * Concatenated
Also add sparsity info to DrakeFunctionConstraint
Add drakeFunction.kinematic.SmoothDistancePenalty

A DrakeFunctionConstraint based on a function of this class  provides the same
functionality as a MinDistanceConstraint.
Add drakeFunction.kinematic.WorldPosition

This convenience function returns a drakeFunction.kinematic.RelativePosition
object.