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

@@ -25,7 +25,6 @@
 #include "Resample/Specular/ComputeFluxScalar.h"
 #include "Sample/Aggregate/IInterference.h"
 #include "Sample/Aggregate/ParticleLayout.h"
-#include "Sample/Interface/LayerInterface.h"
 #include "Sample/Interface/LayerRoughness.h"
 #include "Sample/Material/MaterialBySLDImpl.h"
 #include "Sample/Material/MaterialUtil.h"
@@ -81,14 +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)
-{
-    if (i == 0)
-        return nullptr;
-    return sample.layerInterface(i - 1)->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.
@@ -121,8 +112,8 @@ SliceStack slicify(const MultiLayer& sample, bool useAvgMaterials)
             } else {
                 if (i == nLayers - 1)
                     ASSERT(tl == 0);
-                const LayerRoughness* roughness = layerTopRoughness(sample, i);
-                const double rms = sample.layerInterfaceRMS(i - 1);
+                const LayerRoughness* roughness = sample.layer(i)->roughness();
+                const double rms = sample.layerRoughnessRMS(i);
                 result.addSlice(tl, *material, roughness, rms);
             }
         }
@@ -136,8 +127,8 @@ 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 double rms = (i >= 1) ? sample.layerInterfaceRMS(i - 1) : 0.0;
+        const LayerRoughness* roughness = sample.layer(i)->roughness();
+        const double rms = sample.layerRoughnessRMS(i);
 
         // if no slicing is needed, create single slice for the layer
         if (!particle_span.isFinite()) { // also if layer contains no particles