Discard zeros in sr_gui_hand_calibration

[toliver]

As reported by @jlaforga when the calibration tool averages a few raw values to obtain the one to be used for calibration, it averages one or more zeros.

These zeros are there because the topic /debug_etherCAT_data used by (https://github.com/shadow-robot/sr-ros-interface-ethercat/blob/indigo-devel/sr_robot_lib/python_lib/sr_robot_lib/etherCAT_hand_lib.py) is published before filtering out the bad ethercat frames (full of 0's).

The easiest way to fix this is to discard the zeros in the sr_gui_hand_calibration plugin itself.

[toliver]

The best way to fix it would be to do it when receiving the data from the debug topic here
https://github.com/shadow-robot/sr-ros-interface-ethercat/blob/indigo-devel/sr_robot_lib/python_lib/sr_robot_lib/etherCAT_hand_lib.py#L177

[biofotis]

https://github.com/shadow-robot/sr-ros-interface-ethercat/pull/182/files