add a square flight exclusion zone

def nofly_add(self):
        '''add a square flight exclusion zone'''
        try:
            latlon = self.module('map').click_position
        except Exception:
            print("No position chosen")
            return
        loader = self.wploader
        (center_lat, center_lon) = latlon
        points = []
        points.append(mp_util.gps_offset(center_lat, center_lon, -25,  25))
        points.append(mp_util.gps_offset(center_lat, center_lon,  25,  25))
        points.append(mp_util.gps_offset(center_lat, center_lon,  25, -25))
        points.append(mp_util.gps_offset(center_lat, center_lon, -25, -25))
        start_idx = loader.count()
        for p in points:
            wp = mavutil.mavlink.MAVLink_mission_item_message(0, 0, 0, 0, mavutil.mavlink.MAV_CMD_NAV_FENCE_POLYGON_VERTEX_EXCLUSION,
                                                              0, 1, 4, 0, 0, 0, p[0], p[1], 0)
            loader.add(wp)
        self.loading_waypoints = True
        self.loading_waypoint_lasttime = time.time()
        self.master.mav.mission_write_partial_list_send(self.target_system,
                                                        self.target_component,
                                                        start_idx, start_idx+4)
        print("Added nofly zone")