add fallback std::filesystem implementation

  - minor change to existing code:
    - include <dune/copasi/common/filesystem.hh> instead of <filesystem>
    - use namespace `fs` instead of `std::filesystem`
    - where by default `fs` is just an alias for `std::filesystem`
  - if -DUSE_FALLBACK_FILESYSTEM=ON in CMake configuration
    - `fs` instead uses implementation from https://github.com/gulrak/filesystem
    - no longer links or depends on std::filesystem
    - needed for macOS < 10.15 which doesn't have std::filesystem support