Implement spatially symmetric LoopTNR

[VictorVanthilt]

@dartsushi Implemented spatially symmetric LoopTNR (which i coined SLoopTNR) and I adapted it to fit within this project.
There are three important notes to make here:

1. The internal code uses a different leg-ordering convention (which the user does not have to worry about, use it as you would any other scheme).
2. The implementation is not (yet) compatible with tensors living in special spaces like the classical_ising_symmetric model.
3. The implementation only works for real-valued tensors (and should error if you hand it anything else).