update left and right_virtualspace

[lkdvos]

This is a fix of the long-standing issue #103, which rewrites the package to consistently use left_virtualspace to denote the space to the left of a lattice site, and right_virtualspace to denote the space to the right of a lattice site. Additionally, they are both returned "non-dualised", i.e. left_virtualspace(mps, i + 1) == right_virtualspace(mps, i).

This is obviously very breaking, but should be more consistent in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/MPSKit.jl	100.00% <ø> (ø)
src/algorithms/expval.jl	72.30% <100.00%> (ø)
src/algorithms/fidelity_susceptibility.jl	100.00% <100.00%> (ø)
src/algorithms/timestep/timeevmpo.jl	84.79% <100.00%> (ø)
src/algorithms/toolbox.jl	93.27% <100.00%> (ø)
src/environments/abstract_envs.jl	87.75% <100.00%> (ø)
src/environments/finite_envs.jl	100.00% <100.00%> (+8.06%)	⬆️
src/operators/abstractmpo.jl	39.84% <100.00%> (ø)
src/operators/mpo.jl	94.58% <100.00%> (-0.03%)	⬇️
src/operators/mpohamiltonian.jl	90.22% <100.00%> (ø)
... and 6 more

... and 1 file with indirect coverage changes

[maartenvd]

how do you garantuee left_virtualspace(mps, i + 1) == right_virtualspace(mps, i) during twosite algorithms?

[lkdvos]

Sorry, my comment above was a bit misleading. What I meant is isdual(left_virtualspace(mps, i+1)) == isdual(right_virtualspace(mps, i)), and in most cases (infinite and onesite finite), the spaces are the same as well. In other cases, left_virtualspace(mps, i + 1) will give you the space to the right of the bond between i and i+1, while right_virtualspace(mps, i) gives you the space to the left of that bond. In other words, i now always refers to sites.
As a small remark, it turns out you actually were not really using these functions anyways, and simply calling space on the relevant tensors most of the time, so this has less big of an effect than I initially expected (although I honestly won't be surprised if some bugs still pop up).