Sum/Product Models don't do what they claim

[smk78]

The Sum|Multi model generator is not creating properly parameterised models:

Sum
Should do: scale_factor * {(scale_1 * model_1) + (scale_2 * model_2)} + background

Is doing : scale_factor * {(scale_1 * model_1 + background_1) + (scale_2 * model_2 + background_2)}

Product

Should do: scale_factor * model_1 * model_2 + background

Is doing : (scale_1 * model_1 + background_1) * (scale_2 * model_2 + background_2) + background