Add tests for fermionic periodic boundary conditions

[lkdvos]

Implement fixes for handling fermionic periodic boundary conditions and adds some tests.

I have to say that this still absolutely confuses me, and I can't really wrap my head around this very well.
The bottom line is that if I understand everything correctly, even though our formalism works, it cannot correctly account for the anticommutation relations on different sites:

using MPSKitModels
h_ij = c_plusmin() + c_minplus()
h_ji = permute(h_ij, ((2, 1), (4, 3)))
h_ij ≈ -h_ji # false!!
h_ij ≈ h_ji # true

I don't think there is any way around this, but I might be mistaken here.

In any case, with this in mind, the previous implementation of PBC did not account for this, so here I added a twist on every link that needs to wrap around.

I think the tests should capture this correctly, but I'm still confused by this entire endeavor, since if I am understanding this correctly, for fermionic systems the following does not hold, while it is what I understand from translation invariance for bosonic systems:

H = # some tensor representing a translation invariant hamiltonian
isapprox(H, permute(H, (TupleTools.circshift(codomainind(H), 1), TupleTools.circshift(domainind(H), 1)))

@Jutho if you could confirm that this is indeed expected, that would be greatly appreciated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/transfermatrix/transfer.jl	77.77% <100.00%> (ø)
src/utility/utility.jl	75.94% <100.00%> (-0.31%)	⬇️

... and 1 file with indirect coverage changes

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[leburgel]

Changes themselves look good to me. I think I understand the fix, but I have to say I don't really understand what the problem actually was.