code: def grav_pot(self, x, y, rho0, gamma, center_x=0, center_y=0):
          
          x_ = x - center_x
          y_ = y - center_y
          r = np.sqrt(x_**2 + y_**2)
          mass_3d = self.mass_3d(r, rho0, gamma)
          pot = mass_3d/r
          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: