Applies sum for normal Bases to SBases.
References syten::Basis::dir, and SYTEN_ASSERT_MSG.
Referenced by exp(), syten::MPS::DMRG::Worker::expand_to_left(), syten::MPS::DMRG::Worker::expand_to_right(), syten::MPS::DMRG::Worker::solveLocal(), syten::MPS::DMRG::Worker::solveLocalTwoSite(), and syten::STensorImpl::Autodiff::svd_adjoint_evaluator().