code: def mass_2d(self, R, Rs, rho0):
          
          x = R/Rs
          gx = self.g_(x)
          m_2d = 4*rho0*Rs*R**2*gx/x**2 * np.pi
          return m_2d
docs: mass enclosed a 3d sphere or radius r
          :param r:
          :param Ra:
          :param Rs:
          :return: