Move roughness from LayerInterface to Layer (#20)

MR does the first point of https://jugit.fz-juelich.de/mlz/intern/ba-intern/-/blob/main/internal_reports/2012_Polylayer.md:

Associate surface properties with `Layer`.

Class LayerInterface and API function AddLayerWithTopRoughness were deleted.

GUI/Model/FromCore/ItemizeSample.cpp

@@ -654,7 +654,7 @@ SampleItem* itemizeSample(const MultiLayer& sample, const QString& nodeName)
         const auto* layer = sample.layer(layerIndex);
 
         const LayerRoughness* top_roughness =
-            layerIndex == 0 ? nullptr : sample.layerRoughness(layerIndex);
+            layerIndex == 0 ? nullptr : sample.layer(layerIndex)->roughness();
 
         auto* layerItem = result->createLayerItemAt();
         layerItem->setMaterial(findMaterialItem(matItems, layer));