def add_parameter_subscriber(subscriber)
      subscriber.set_manager(self)
      @parameter_subscribers.push(subscriber)
      @master.subscribe_param(subscriber.key)
      subscriber
    end
def add_parameter_subscriber(subscriber)
      subscriber.set_manager(self)
      @parameter_subscribers.push(subscriber)
      @master.subscribe_param(subscriber.key)
      subscriber
    end
register callback for paramUpdate
 @param [ParameterSubscriber] subscriber ParameterSubscriber instance to be added
 @return [ParameterSubscriber] subscriber