code: def get_center_of_mass(self):
          
          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)
docs: 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.