Add functions in JointPositionBinaryCommandSender to get off_pos_ and set on_pos_ #687

Job	Run time
industrial_ci (noetic, testing)	10m 48s
industrial_ci (noetic, main)	8m 20s
industrial_ci (melodic, testing)	8m 42s
industrial_ci (melodic, main)	8m 24s
	36m 14s

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