Add Hubbard model

[lkdvos]

This overloads the Hubbard model from MPSKitModels.jl

TODO:

• find some benchmarkdata to test
• implement fixed particle number

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/models.jl	0.00%	6 Missing ⚠️
src/operators/lattices/squarelattice.jl	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/operators/lattices/squarelattice.jl	95.00% <0.00%> (-5.00%)	⬇️
src/operators/models.jl	70.58% <0.00%> (-9.42%)	⬇️

[Yue-Zhengyuan]

Why is fixed particle number still not implemented? I think there is already FermionNumber symmetry class。

For benchmarking, maybe you can have a look at https://yastn.github.io/yastn/quickstart/hubbard_purification.html#d-fermi-hubbard-model-at-finite-temperature

[lkdvos]

I should probably be a little more careful with what I meant. Indeed, there is no problem using this function with imposing FermionNumber, which is just a combination of U1 and FermionParity. The subtlety lies in how symmetric tensors work: all tensors always have to have total charge 0. In essence, this means that imposing FermionNumber automatically makes you impose total particle number 0.
It is definitely possible to select a different filling, but this requires injection of U1 charges per site, typically also resulting in unit cells, and while I know how to achieve this, I have not yet figured out a good interface to do this.