New goal programming keeping soft constraints around

• When keep_soft_constraints is True, we append a non-goal GoalConstraint to self.__problem_constraints. This coulds be seen as a bit hacky, as there is no one goal associated with it (but a bunch of them). It does make sure that order of constraints is the same (and everything in constraints() is handled uniformly). Should it be a separate list, or is it OK this way? Would order of variables/constraints even matter?
• Add seed() for objective linearization variables?
• new_goal_programming_mixin branch scaled the constraint with the number of goals. We don't do that anymore (also not in merge_goal_programming_mixin branch), but should we? --> We shouldn't. Current way is fine.