Fix const correctness issues

[aditya-vk]

Revisit aikido/include/robot/* to ensure const-correctness after the planner API is in place and is const-correct.

---

• https://github.com/personalrobotics/aikido/pull/314/files#diff-a65ad42402fcdc572aad061dc655cabfR21

[aditya-vk]

1. Any place that takes in a statespace, needs to take a const statespace. For instance, planner and robot namespaces are affected by this, example interpolator takes statespace. Take it by const.
2. In Hand class, edit the docstring for getters of body nodes.

[brianhou]

Now that the planner API is mostly complete, we should consider revisiting this issue.