added convenience function to build layered atmosphere as in C7

Environment/Convenience.h

+/*
+ * (c) Copyright 2019 CORSIKA Project, corsika-project@lists.kit.edu
+ *
+ * See file AUTHORS for a list of contributors.
+ *
+ * This software is distributed under the terms of the GNU General Public
+ * Licence version 3 (GPL Version 3). See file LICENSE for a full version of
+ * the license.
+ */
+
+#include <corsika/environment/NuclearComposition.h>
+#include <corsika/environment/FlatExponential.h>
+#include <corsika/environment/SlidingPlanarExponential.h>
+#include <corsika/environment/VolumeTreeNode.h>
+#include <corsika/particles/ParticleProperties.h>
+#include <corsika/units/PhysicalUnits.h>
+
+#include <memory>
+
+namespace corsika::environment {
+
+class LayeredAtmosphereBuilder {
+    std::unique_ptr<NuclearComposition> nuclearComposition_;
+    geometry::Point center_;
+    units::si::LengthType previousRadius_{LengthType::zero()};
+    static auto constexpr earthRadius_ = 6371_km;
+    
+public:
+    LayeredAtmosphereBuilder(corsika::geometry::Point center) : center_(center) {}
+    
+    void setNuclearComposition(NuclearComposition composition) {
+        nuclearComposition_ = std::make_unique<NuclearComposition>(composition);
+    }
+    
+    void addLayer(units::si::GrammageType a, units::si::GrammageType b, units::si::LengthType c, units::si::LengthType thickness) {
+        auto const radius = previousRadius_ + thickness;
+        
+        std::make_unique<BaseNodeType>(
+          std::make_unique<geometry::Sphere>(center_, radius);
+          
+        previousRadius_ = radiusl
+    }
+    
+    auto const assemble() {
+        // get the outmost VolumeTreeNode
+    }
+};
+
+}