load waypoints from a file.
        returns number of waypoints loaded

def load(self, filename):
        '''load waypoints from a file.
        returns number of waypoints loaded'''
        f = open(filename, mode='r')
        version_line = f.readline().strip()
        if version_line == "QGC WPL 100":
            readfn = self._read_waypoints_v100
        elif version_line == "QGC WPL 110":
            readfn = self._read_waypoints_v110
        elif version_line == "QGC WPL PB 110":
            readfn = self._read_waypoints_pb_110
        else:
            f.close()
            raise MAVWPError("Unsupported waypoint format '%s'" % version_line)

        self.clear()
        readfn(f)
        f.close()

        return len(self.wpoints)