Allow for split actuator or combined x- and y-arm actuation for stages

Make sure pyDARM can handle a split actuator scheme where the DARM LSC signal is fed back to say, x-arm L1, L2 stages and y-arm L3 stage. [Note, I think pyDARM can already handle this]

Alternatively, we might also end up running with feedback signals running to both x- and y-arm on the same stage someday. For example, the feedback signal might go to both x- and y-arm L3 stage. [I think pyDARM cannot do this at present].