Calculates the squared norm of the state with respect to the current two focus sites, i.e. completes the double layer.
References syten::abs(), bsite, cb, cbb, syten::clear(), ct, ctb, syten::imag(), lb, lbb, syten::logGD(), lt, ltb, rb, rbb, syten::real(), rt, rtb, std::scientific(), std::setprecision(), syten::IPEPS::sldlmap(), and tsite.
Referenced by expectation_value(), and norm().