Calculates the squared norm of the state with respect to the current two focus sites, i.e. completes the double layer.
References syten::abs(), bl, blb, br, brb, cl, clb, cr, crb, syten::imag(), syten::logGD(), lsite, syten::real(), rsite, std::scientific(), std::setprecision(), syten::IPEPS::sldlmap(), tl, tlb, tr, and trb.
Referenced by expectation_value(), and norm().