Improve fmodel

[johannes-moegerle]

• improve the fModel struct to directly store all relevant infos like f_tot and nu_range
• add get_fmodels functionality, see below

Example getting all relevant models up to n_max

The idea is to now be able to get all relevant models (including LOWN, HIGHN andauto generated single channel models) dynamically as follows:

n_max = 30

species = :Yb174
parameters = MQDT.get_species_parameters(species)

all_models = Vector{MQDT.fModel}()

sr = 1 / 2
Jc = 1 / 2
ic = parameters.spin
for lr in 0:(n_max - 1)
    for Jr in abs(lr - sr):1:(lr + sr)
        for Fc in abs(Jc - ic):1:(Jc + ic)
            for F in abs(Fc - Jr):1:(Fc + Jr)
                models = MQDT.get_fmodels(species, lr, Jr, Fc, F, parameters)
                for model in models
                    if !any(m -> m.name == model.name, all_models)
                        push!(all_models, model)
                    end
                end
            end
        end
    end
end

see also the updated file examples/generate_Yb174_database.jl or the PR for database-mqdt