Add t-J model

[Yue-Zhengyuan]

Currently only Abelian symmetries are implemented:

• Particle: Trivial and U1Irrep
• Spin: Trivial and U1Irrep

There is a sf::Bool slave_fermion::Bool option that switches to the slave-fermion basis of the physical space, in which the holons are fermionic while spinons are bosonic.

[codecov]

Codecov Report

Attention: Patch coverage is 77.46914% with 73 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/tjoperators.jl	80.96%	59 Missing ⚠️
src/models/hamiltonians.jl	0.00%	14 Missing ⚠️

Files with missing lines	Coverage Δ
src/operators/hubbardoperators.jl	69.72% <ø> (ø)
src/models/hamiltonians.jl	19.35% <0.00%> (-2.47%)	⬇️
src/operators/tjoperators.jl	80.96% <80.96%> (ø)

[lkdvos]

Overall looks good, thanks a lot!

Some minor remarks I would like to change before merging:

• Could you try to revert the formatting changes you made that are unrelated to the changes of this PR? I'm not necessarily attached to the formatting style, but it's a bit nicer to keep that in separate commits/PRs
• Can you maybe expand sf into slavefermion? I think this is slightly clearer.
• Similarly, I think e_plusmin_updown is slightly clearer than e_plusmin_ud, but this is less of a strong opinion.
• In docstrings, there is typically a distinction between code in single backticks, and math in double backticks. I don't mind too much if it is LaTeX or unicode, but I would avoid having LaTeX curly braces in single backticks, as that gets parsed/highlighted differently

[Yue-Zhengyuan]

@lkdvos I have made the requested changes; please see if everything is OK now

[lkdvos]

Changed some small formatting things, will merge once tests finish. Thanks!