code: def grav_pot(self, x, y, rho0, Rs, center_x=0, center_y=0):
          
          x_ = x - center_x
          y_ = y - center_y
          r = np.sqrt(x_**2 + y_**2)
          M = self.mass_tot(rho0, Rs)
          pot = M / (r + Rs)
          return pot
docs: gravitational potential (modulo 4 pi G and rho0 in appropriate units)
          :param x:
          :param y:
          :param rho0:
          :param a:
          :param s:
          :param center_x:
          :param center_y:
          :return: