Interface: Pack `(bra, ket)` into one tuple for `expectation_value`

[Yue-Zhengyuan]

Currently, in expectation_value and reduced_densitymatrix:

• For iPEPS measurement, we can provide either a single ket::InfinitePEPS, or bra::InfinitePEPS and ket::InfinitePEPS as two separate arguments.
• For purified iPEPO, we always provide bra::InfinitePEPO and ket::InfinitePEPO.
• For one-layer iPEPO, we provide only one ρ::InfinitePEPO.

So we have either one or two arguments to specify the states. Do we want to unify the interface to one state argument by packing (bra, ket) into one tuple?