code: def mass_3d_lens(self, R, Rs, theta_Rs):
          
          rho0 = self._alpha2rho0(theta_Rs, Rs)
          m_3d = self.mass_3d(R, Rs, rho0)
          return m_3d
docs: mass enclosed a 3d sphere or radius r
          :param r:
          :param Ra:
          :param Rs:
          :return: