Add convenience function to retrieve specific groups of factors

[ConnectedSystems]

It would be useful to retrieve parameter groups from the model spec.

Tentative implementation:

function extract_param_group_idx(model_spec::DataFrame, needle::String)::Vector{Int64}
    needle_pos = contains.(string.(model_spec.fieldname), needle)
    return findall(needle_pos)
end
function extract_param_group_idx(model::Model, component, needle::String)::Vector{Int64}
    comp_params = component_params(model, component)
    return extract_param_group_idx(comp_params, needle)
end

function extract_param_group(model::Model, component, needle::String)::DataFrame
    group_pos = extract_param_group_idx(model, component, needle)
    comp_params = component_params(model, component)
    return comp_params[group_pos, :]
end
function extract_param_group(model_spec::DataFrame, needle::String)::DataFrame
    group_pos = extract_param_group_idx(model_spec, needle)
    return model_spec[group_pos, :]
end