docs: def center(self, weights=None):
          """ return the center of each object """
          if weights is None:
              weights = self.data.weights
          mass = utils.bincount(self.labels, weights, self.N)
          cp = numpy.empty((len(mass), self.data.pos.shape[-1]), 'f8')
          for d in range(self.data.pos.shape[-1]):
              cp[..., d] = utils.bincount(self.labels, weights *
                      self.data.pos[..., d], self.N)
              cp[..., d] /= mass
          return cp