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.

CHANGELOG

@@ -5,6 +5,7 @@ BornAgain-22.0, in preparation
     * Datafield::flat flattens out axis of size 1
    > API:
     * Removed RectangularDetector.
+    * Removed AddLayerWithTopRoughness. Roughness now belongs to the layer itself.
     * Offspec analyzer is set for detector, not scan(#651)
     * FitObjective::addSimulationAndData has been replaced by function addFitPair,
       which takes experimental data in form of a Datafield object instead of a NumPy array