Get a tuple (x,y) that is the center of mass. The center of mass is not
        necessarily the same as the center of the bounding box. Imagine a black
        square and a single dot wide outside of the square.

def get_center_of_mass(self):
        """
        Get a tuple (x,y) that is the center of mass. The center of mass is not
        necessarily the same as the center of the bounding box. Imagine a black
        square and a single dot wide outside of the square.
        """
        xsum, ysum, counter = 0., 0., 0
        for stroke in self.get_pointlist():
            for point in stroke:
                xsum += point['x']
                ysum += point['y']
                counter += 1
        return (xsum / counter, ysum / counter)