Implement fixed particle number density for the `hubbard_model` Hamiltonian

[leburgel]

The implementation of MPSKitModels.hubbard_model on a PEPSKit.InfiniteSquare lattice currently does not support a fixed particle number density. There is an example on how to do this manually, but we should support specifying uniform fillings when calling MPSKitModels.hubbard_model directly.

This should be addressed after it's been impelemented in MPSKitModels.jl itself (see QuantumKitHub/MPSKitModels.jl#47).