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.

Resample/Processed/ReSample.cpp

@@ -80,14 +80,6 @@ std::vector<ZLimits> particleSpans(const MultiLayer& sample)
     return result;
 }
 
-//! Returns top roughness of layer
-const LayerRoughness* layerTopRoughness(const MultiLayer& sample, size_t i_layer)
-{
-    if (i_layer == 0)
-        return nullptr;
-    return sample.layer(i_layer)->roughness();
-}
-
 //! Returns a vector of slices that represent the layer structure of the sample.
 //!
 //! Each slice is either a layer, or a fraction of a layer.
@@ -120,7 +112,7 @@ SliceStack slicify(const MultiLayer& sample, bool useAvgMaterials)
             } else {
                 if (i == nLayers - 1)
                     ASSERT(tl == 0);
-                const LayerRoughness* roughness = layerTopRoughness(sample, i);
+                const LayerRoughness* roughness = sample.layer(i)->roughness();
                 const double rms = sample.layerRoughnessRMS(i);
                 result.addSlice(tl, *material, roughness, rms);
             }
@@ -135,7 +127,7 @@ SliceStack slicify(const MultiLayer& sample, bool useAvgMaterials)
         double tl = layer->thickness();
         const Material* const material = layer->material();
         const ZLimits& particle_span = particle_spans[i];
-        const LayerRoughness* roughness = layerTopRoughness(sample, i);
+        const LayerRoughness* roughness = sample.layer(i)->roughness();
         const double rms = sample.layerRoughnessRMS(i);
 
         // if no slicing is needed, create single slice for the layer