Relax MPO element type restrictions

[leburgel]

Attempt at relaxing the type restrictions on MPO elements, in order to be able to use generic effective MPOs. This should work as long as the element type adheres to the (until now implicit) interface for MPOTensor which requires:

• left_virtualspace
• right_virtualspace
• _conj_mpo
• add_physical_charge
• _fuse_mpo_mpo
• _fuse_mpo_mps
• _mpo_to_mps
• ...?

I also added explicit type annotations for internal methods where an element type <:MPOTensor is really assumed.

See also QuantumKitHub/PEPSKit.jl#125 for the intended use case.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/mpo.jl	81.81%	2 Missing ⚠️
src/operators/multilinempo.jl	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/operators/abstractmpo.jl	50.63% <100.00%> (+0.31%)	⬆️
src/operators/multilinempo.jl	35.00% <50.00%> (ø)
src/operators/mpo.jl	90.34% <81.81%> (-0.32%)	⬇️

... and 1 file with indirect coverage changes

[lkdvos]

Overall, looks clean to me. Can you confirm that the PEPSKit changes you want have passing tests before I merge and tag any of this?