got rid of std::filesystem as it is not available with g++ 7

Framework/Utilities/CorsikaData.cc

+/*
+ * (c) Copyright 2020 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/utl/CorsikaData.h>
 
 #include <cstdlib>
 #include <stdexcept>
 #include <string>
 
-std::filesystem::path corsika::utl::CorsikaData(std::filesystem::path const& key) {
+std::string corsika::utl::CorsikaData(std::string const& key) {
   if (auto const* p = std::getenv("CORSIKA_DATA"); p != nullptr) {
-    auto const path = std::filesystem::path(p) / key;
+    auto const path = std::string(p) + "/" + key;
     return path;
   } else {
     throw std::runtime_error("CORSIKA_DATA not set");

Framework/Utilities/CorsikaData.h

@@ -8,12 +8,12 @@
  * the license.
  */
 
-#include <filesystem>
+//#include <filesystem>
 #include <string>
 
 namespace corsika::utl {
   /**
    * returns the full path of the file \p filename within the CORSIKA_DATA directory
    */
-  std::filesystem::path CorsikaData(std::filesystem::path const& filename);
+  std::string CorsikaData(std::string const& filename);
 } // namespace corsika::utl

Processes/UrQMD/UrQMD.cc

@@ -31,7 +31,7 @@ using SetupStack = corsika::setup::Stack;
 using SetupParticle = corsika::setup::Stack::StackIterator;
 using SetupProjectile = corsika::setup::StackView::StackIterator;
 
-UrQMD::UrQMD(std::filesystem::path const& xs_file) {
+UrQMD::UrQMD(std::string const& xs_file) {
   readXSFile(xs_file);
   iniurqmd_();
 }
@@ -435,11 +435,11 @@ std::pair<int, int> corsika::process::UrQMD::ConvertToUrQMD(
   return mapPDGToUrQMD.at(static_cast<int>(GetPDG(code)));
 }
 
-void UrQMD::readXSFile(std::filesystem::path const& filename) {
+void UrQMD::readXSFile(std::string const& filename) {
   std::ifstream file(filename, std::ios::in);
 
   if (!file.is_open()) {
-    throw std::runtime_error(filename.native() + " could not be opened.");
+    throw std::runtime_error(filename + " could not be opened.");
   }
 
   std::string line;

Processes/UrQMD/UrQMD.h

@@ -21,15 +21,14 @@
 #include <boost/multi_array.hpp>
 
 #include <array>
-#include <filesystem>
 #include <random>
+#include <string>
 #include <utility>
 
 namespace corsika::process::UrQMD {
   class UrQMD : public corsika::process::InteractionProcess<UrQMD> {
   public:
-    UrQMD(
-        std::filesystem::path const& path = utl::CorsikaData("UrQMD/UrQMD-1.3.1-xs.dat"));
+    UrQMD(std::string const& path = utl::CorsikaData("UrQMD/UrQMD-1.3.1-xs.dat"));
     void Init() {}
     corsika::units::si::GrammageType GetInteractionLength(
         corsika::setup::Stack::StackIterator const&) const;
@@ -51,7 +50,7 @@ namespace corsika::process::UrQMD {
     bool CanInteract(particles::Code) const;
 
   private:
-    void readXSFile(std::filesystem::path const&);
+    void readXSFile(std::string const&);
 
     corsika::random::RNG& fRNG =
         corsika::random::RNGManager::GetInstance().GetRandomStream("UrQMD");