Ignore nan throttle, velocity, pos

[tabiosg]

Title for motors

Ok imagine IK. Currently we send an entire arm_position_cmd. But that includes allen key and gripper.

Allen key and gripper needs to be sent nans, and we need to ignore those

[qhdwight]

I would prefer just allowing any subset of the motor group, then sending one subset to arm_position_cmd and another to arm_throttle_cmd