Starts (Subscribes) the client.

def start (self):
        '''
        Starts (Subscribes) the client.

        '''
        self.subrgb = rospy.Subscriber(self.topicrgb, ImageROS)
        self.subd = rospy.Subscriber(self.topicd, ImageROS)
        self.ts = message_filters.ApproximateTimeSynchronizer([subrgb, subd], 1, 0.1, allow_headerless=True)
        self.ts.registerCallback(self.__callback)