explicitly convert path to string

  - mingw gcc on msys will not implicitly convert std::filesystem::path to std::string
  - `cannot convert 'path' (type 'const std::filesystem::__cxx11::path') to type 'const string&' {aka 'const std::__cxx11::basic_string<char>&'}`
  - https://stackoverflow.com/a/57377681/6465472

dune/copasi/model/diffusion_reaction.cc

@@ -319,7 +319,7 @@ ModelDiffusionReaction<Traits>::setup_vtk_writer()
     auto writer =
       std::make_shared<VTKWriter<VTKGridView>>(gfs->gridView(), Dune::VTK::conforming);
     auto sequential_writer =
-      VTKSequenceWriter<VTKGridView>{ writer, name, path, path };
+      VTKSequenceWriter<VTKGridView>{ writer, name, path.string(), path.string() };
     sequential_writer.setTimeSteps(timesteps);
 
     using Predicate = PDELab::vtk::DefaultPredicate;

dune/copasi/model/multidomain_diffusion_reaction.cc

@@ -307,7 +307,7 @@ ModelMultiDomainDiffusionReaction<Traits>::setup_vtk_writer()
       auto writer = std::make_shared<VTKWriter<SubDomainGridView>>(
         sub_grid_view, Dune::VTK::conforming);
 
-      auto sequential_writer = VTKSequenceWriter{ writer, name, path, path };
+      auto sequential_writer = VTKSequenceWriter{ writer, name, path.string(), path.string()};
       sequential_writer.setTimeSteps(timesteps);
 
       auto collector =