Clean up ConcreteRobot plan methods to take MetaSkeletonStateSpace::State*.

[sniyaz]

During the review of #588, we realized that (for some reason) these methods take StateSpace::State* instead of MetaSkeletonStateSpace::State*. Since we're working with ConcreteRobot we must be working with MetaSkeletonStateSpace::State* (unless the user is doing something very stupid).

We should fix this at some point, potentially during @aditya-vk's refactor of the Robot classes.