Uniform return types for backend features

# Feature Request

As an end-user, I want any given implementation of a fixed backend feature like `PlanMotion` to return the same type across all backend clients (`JointTrajectory` in the `PlanMotion` case) so that I don't have to think so hard.

## Details

After the backend features refactoring, the backend feature interfaces include documentation suggesting return types for each feature. Currently the V-REP implementations do not always adhere to these suggestions. For a more consistent UX, the V-REP should be refactored to meet this new standard.