Fixed a bug with MaskLayer not having a target mask layer when using an index for maskMethod
Fixed a bug with MaskLayer not having a target mask layer when using an index for maskMethod

dpaiton/OpenPV,PetaVision/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,PetaVision/OpenPV,PetaVision/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV,dpaiton/OpenPV