Added a new helper method to update the end-effector link for control

[DasGuna]

A single method has been added called set_end_effector_link, which has a default argument of None (causing exit if not specified - retaining the default link without any change). If not None, calls the move_group.set_end_effector_link method and updates the arm_commander self.END_EFFECTOR_LINK variable.

Please note functional tests below:

• Confirmed updated link for end-effector works (i.e., move to pose reflected on changed link)
• Confirmed empty request and exit (without change). Debugging message as ROS warn log level
• Tested non-existent link input. Currently terminates the node as TF fails to lookup. Future work here needs to get a list of valid links in tree and confirm prior to setting. TODO added in code as reference.