Add param 'locked' to control startup behaviour

If true will try to hold end-effector position at origin. To
unclock, grey and white buttons have to be pressed simultaneously.
Since both buttons are only available on Omni, 'locked' is set to
'false' by default.
Remove unused variable
Rename class variables according to ROS guidelines
Add support for force feedback message in user frame
Merge branch 'feature-force-frame'

Changes force feedback interface to geometry_msgs::WrenchStamped. So
force and torque vectors will be transformed from user frame to Phantom
force feedback frame, i.e. to sensable_origin.