Replace VehicleTrailer/DiffSteerTrailer with an ArticulatedVehicle (or delete them) #773

Trailer, DiffSteerTrailer and VehicleTrailer in src/roboticstoolbox/mobile/Vehicle.py were a late addition, are not used in the book, and are in poor shape. They are not exported from roboticstoolbox, have no tests and are not in the docs, so only code that imports them from roboticstoolbox.mobile.Vehicle can be affected.

Found during the type-hinting pass. They were annotated only to the extent needed for mypy to pass; their defects were recorded, not fixed.

Proposal

Investigate replacing VehicleTrailer and DiffSteerTrailer with a single ArticulatedVehicle class that is a proper VehicleBase subclass and composes a Trailer. Drop DiffSteerTrailer. This is a design question, not a patch.

Known defects (headstart)

• VehicleTrailer does not call super().__init__, so most VehicleBase attributes (_V, _control, ...) do not exist on it; _x0 is not converted to an array; step returns None where the base returns odometry (hence # type: ignore[override]). Needs a decision: composition, or a proper subclass.

• VehicleTrailer.plot is a copy of DiffSteerTrailer.plot and uses self._L1, self._L2, self._animation_trailer, none of which exist on VehicleTrailer. Dead code.

• DiffSteerTrailer.deriv has a leftover print(X[2], eta, etad) and a dead if X[2] > 0.01: pass; DiffSteerTrailer.__init__ calls x0.copy(), which only works because the default is a list.

• Trailer._animation was only set when an animation or polygon was given, but VehicleTrailer.init reads it (initialised to None in the typing PR; it crashed before).

• DiffSteerTrailer.init called self._animation_trailer.add even when trailer=None (now guarded).

• Trailer.__init__ has placeholder docstring text (_summary_, _description_).

• The runblock example in the VehicleTrailer.step docstring uses VehicleTrailer and Trailer without importing them (a NameError when the docs are built)

Acceptance

• Decide: ArticulatedVehicle or delete.
• Whatever remains has tests, docs and a docstring, and VehicleBase attributes are all initialised.