Split Models and Fields more

[ffreyer]

As things are now, the field represents most of the interaction, with only the prefactor being held by the model. The model mainly produces the hopping matrix and contains the lattice. It may make sense to separate these components more, specifically the field as it is the main driver of complexity.

For example:

struct HubbardModel <: Model
    lattice::AbstractLattice # maybe move this to DQMC struct?
    hopping::GenericHoppings
    interaction::HubbardInteraction
end

struct GenericHoppings
    # use bond labels as key to this dict (with some default key fallback)
    parameters::Dict{Symbol, Number}
end

struct HubbardInteraction
    U::Float64
end

struct SomeHubbardField
    interaction::HubbardInteraction

    internals...
end

function field_constructor(parameters, interaction::HubbardInteraction)
    ...
end

This would add HubbardInteraction as a proxy for creating a field. Making a new model with a Hubbard interaction could just reuse that type and not deal with anything field related.