Fix docstrings in StateWrapper, CarWrapper, and PhysicsWrapper

[mrkvicka22]

More specifically, the methods StateWrapper.format_state(), PhysicsWrapper._encode() and PhysicsWrapper._encode() state that they return strings when in fact they return python lists. Note: Other methods that deal with decoding/encoding most likely also have faulty docstrings.

Maybe they should be left as numpy arrays for performance reasons unless it makes sense to keep them as lists.