Various filesystem refactorings

– DataTree::packageDir() now takes a std::string_view, returns a
  std::filesystem::path, and no longer creates that directory
– DataTree::writeToFileIfModified() now takes a std::filesystem::path as
  argument
– Do not call DataTree::writeToFileIfModified() for generating MATLAB/Octave
  files, since it does not work (the directory inside which the file is written
  has been deleted by the preprocessor just before)
– Consistently use DataTree::packageDir() everywhere (for compatibility with
  planner_objective)