docs: def _distance(self, x0, y0, x1, y1):
          """Utitlity function to compute distance between points."""
          dx = x1-x0
          dy = y1-y0
          # roll displacements across the borders
          if self.pix:
              dx[ dx > self.Lx/2 ] -= self.Lx
              dx[ dx < -self.Lx/2 ] += self.Lx
          if self.piy:
              dy[ dy > self.Ly/2 ] -= self.Ly
              dy[ dy < -self.Ly/2 ] += self.Ly
          return dx, dy