def _calculate_average(self, points):
        """Calculate the arithmetic mean of the points x and y coordinates
           seperately.
        """
        assert len(self.theta) == len(points), \
            "points has length %i, but should have length %i" % \
            (len(points), len(self.theta))
        new_point = {'x': 0, 'y': 0, 'time': 0}
        for key in new_point:
            new_point[key] = self.theta[0] * points[0][key] + \
                self.theta[1] * points[1][key] + \
                self.theta[2] * points[2][key]
        return new_point
Calculate the arithmetic mean of the points x and y coordinates
           seperately.