Fix const correctness in multiple places

[jslee02]

At a glance, there are several places violating const correctness like a const function returning a non-const pointer to the member variable.

For example:

• https://github.com/personalrobotics/aikido/blob/master/include/aikido/statespace/StateSpace.hpp#L107

• https://github.com/personalrobotics/aikido/blob/master/include/aikido/trajectory/Trajectory.hpp#L24

• https://github.com/personalrobotics/aikido/blob/master/include/aikido/control/KinematicSimulationTrajectoryExecutor.hpp#L56

• Many functions take const pointers and not pointers to const objects. For example const trajectory::TrajectoryPtr when the intention is trejctory::ConsTrajectoryPtr.

However, this issue cannot be fixed now because it will cause other problems that should be fixed by upcoming changes like #278. So this should be fixed once those changes are made.