GoalProgrammingMixin: scale_by_problem_size time step scaling behaviour not appropriate for sparse path goals
scale_by_problem_size divides the path objective function by the number of time steps.
This is fine normally. But not in situations where we have a path goal, where a target value is specified for a handful of points only.
Consider scaling with the number of time steps for which a target value is set, instead of scaling with the total number of time steps.