Callback function to receive and save Bumper Scans. 

        @param event: ROS BumperScan received
        
        @type event: BumperScan

def __callback (self, event):
        '''
        Callback function to receive and save Bumper Scans. 

        @param event: ROS BumperScan received
        
        @type event: BumperScan

        '''
        bump = bumperEvent2BumperData(event)

        if bump.state == 1:
            self.lock.acquire()
            self.time = current_milli_time()
            self.data = bump
            self.lock.release()