default active nodes in step rather than in init
default active nodes in step rather than in init

otherwise it only works if the nodenet is initialized with all sensors present.
plus, stop stepping if all nodes have been treated (and thus, active_nodes being empty)

printedheart/micropsi2,ianupright/micropsi2,printedheart/micropsi2,ianupright/micropsi2,ianupright/micropsi2,printedheart/micropsi2