added missing file

Environment/IEmpty.hpp

+/*
+ * (c) Copyright 2020 CORSIKA Project, corsika-project@lists.kit.edu
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <corsika/units/PhysicalUnits.h>
+#include <corsika/geometry/Trajectory.h>
+
+namespace corsika::environment {
+
+  /**
+   * \class IEmpty
+   *
+   * intended for usage as default template argument for environments
+   * with no properties.  For now, the ArclengthFromGrammage is
+   * mandatory, since it is used even in the most simple Cascade code.
+   *
+   * - IEmpty is the interface definition.
+   * - Empty<IEmpty> is a possible model implementation
+   *
+   **/
+
+  class IEmpty {
+  public:
+    virtual corsika::units::si::LengthType ArclengthFromGrammage(
+        corsika::geometry::LineTrajectory const&,
+        corsika::units::si::GrammageType) const = 0;
+  };
+
+  template <typename TModel = IEmpty>
+  class Empty : public TModel {
+  public:
+    corsika::units::si::LengthType ArclengthFromGrammage(
+        corsika::geometry::LineTrajectory const&,
+        corsika::units::si::GrammageType) const {
+      return 0. * corsika::units::si::meter;
+    }
+  };
+
+} // namespace corsika::environment