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

@@ -27,7 +27,6 @@
 #include "Sample/Aggregate/Interferences.h"
 #include "Sample/Aggregate/ParticleLayout.h"
 #include "Sample/HardParticle/HardParticles.h"
-#include "Sample/Interface/LayerInterface.h"
 #include "Sample/Interface/LayerRoughness.h"
 #include "Sample/Multilayer/Layer.h"
 #include "Sample/Multilayer/MultiLayer.h"
@@ -302,15 +301,13 @@ void set_CrosscorrelationModel(RoughnessItem* parent, const CrosscorrelationMode
             cd->baseCrossCorrDepth(), cd->baseSpatialFrequency(), cd->power()));
 }
 
-void set_Roughness(LayerItem* parent, const LayerInterface* top_interface)
+void set_Roughness(LayerItem* parent, const LayerRoughness* roughness)
 {
-    if (!top_interface) {
+    if (!roughness) {
         parent->roughnessSelection().setCertainItem(nullptr);
         return;
     }
 
-    const LayerRoughness* roughness = top_interface->roughness();
-
     if (!roughness->showInScriptOrGui()) {
         parent->roughnessSelection().setCertainItem(nullptr);
         return;
@@ -656,15 +653,15 @@ SampleItem* itemizeSample(const MultiLayer& sample, const QString& nodeName)
     for (size_t layerIndex = 0; layerIndex < sample.numberOfLayers(); layerIndex++) {
         const auto* layer = sample.layer(layerIndex);
 
-        const LayerInterface* top_interface =
-            layerIndex == 0 ? nullptr : sample.layerInterface(layerIndex - 1);
+        const LayerRoughness* top_roughness =
+            layerIndex == 0 ? nullptr : sample.layer(layerIndex)->roughness();
 
         auto* layerItem = result->createLayerItemAt();
         layerItem->setMaterial(findMaterialItem(matItems, layer));
         layerItem->thickness().setDVal(layer->thickness());
         layerItem->setNumSlices(layer->numberOfSlices());
 
-        set_Roughness(layerItem, top_interface);
+        set_Roughness(layerItem, top_roughness);
 
         // iterate over particle layouts
         for (const auto* layout : layer->layouts()) {