Fixing Missing End Effector Link for UR-X URDF models - #167

Grippers and end effectors are still WIP, some thoughts on the Wiki https://github.com/petercorke/robotics-toolbox-python/wiki/Grippers

Thank you for your interest in RTB, and for catching this — missing end-effector links causing a crash on fkine() was a real problem.

This PR (#167) predates a large amount of restructuring since 2021: the base class you're working against here (ERobot, with urdf_to_ets_args()) no longer exists — ERobot is now Robot/URDFRobot, loaded via URDFRobot.__init__(name, ..., gripper_link_index=N) directly, so this specific diff can't apply as written.

The underlying concern is partially addressed today: UR5 already passes gripper_link_index=7. UR3/UR10 still don't — worth a fresh look with today's model classes if you're up for it, but closing this one given how much has moved underneath it. Thanks again for the original catch.