Computes the overlap of two operators.
References syten::STensorImpl::ferm_trace(), overlap_rec(), and syten::T3N::Tree::Tree< T >::Root.
Referenced by syten::T3N::RDMs::OneParticleRDM::construct(), norm(), syten::T3N::VarationalTrunc::run(), syten::T3N::RDMs::OneParticleRDM::twoSite_right_rec(), and syten::T3N::RDMs::OneParticleRDM::twoSite_upwards_rec().