RoughnessModel: from GUI to script and back

Following !2703 (merged)

Now roughness script generated by GUI looks like

    # Define roughness
    interlayer_1 = ba.ErfRoughness()
    interlayer_2 = ba.TanhRoughness()

    roughness_1 = ba.LayerRoughness(3*nm, 0.78, 65*nm, interlayer_1)
    roughness_2 = ba.LayerRoughness(2*nm, 0.88, 25*nm, interlayer_2)