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.

Sample/Multilayer/Layer.cpp

@@ -23,7+23,7 @@
 LayerRoughness* zeroRoughness()
 {
     K_CorrelationModel autocorr(0);
     ErfInterlayer interlayer;
     return new LayerRoughness(&autocorr, &interlayer);
 }
 
@@ -38,7+38,7 @@
     , m_thickness(thickness)
     , m_roughness(roughness ? roughness->clone() : zeroRoughness())
 {
+    // If the roughness is not defined by user, it is equivalent to the situation when roughness is
+    // defined, but has zero rms. To avoid constant nullptr checks in the code and to ease
+    // resampling it was accepted that "Layer" should always have non-null roughness descriptor.
     ASSERT(m_roughness);
     if (thickness < 0.)
         throw std::runtime_error("Layer contructor called with negative thickness");