Change parameter and topic names to relative in etherCAT_hand_lib.py

[toliver]

In sr_robot_lib/python_lib/etherCAT_hand_lib.py

make all the names relative (without the initial /) so that we can use the library to control more than one hand.

The idea here is that every hand will be plugged to its own eth port, and control by its own realtime node. Every RT node and all its associated tools (GUI, robot monitor, etc) will be launched in a separate namespace, e.g. NS="right_hand" and NS="left_hand".