Returns the basis of the proxied object on the which
-th leg.
Flips the basis direction if the tensor is conjugated and adds prime_offsets[which-1]
to its prime level.
References conjugate, syten::STensorImpl::SBasis::flip(), name_overwrites, syten::STensorImpl::SBasis::prime(), prime_offsets, and syten::STensorImpl::SBasis::rename().
Referenced by parity().