code: def export_path(self, relname, export_dir=None):
          
          # removing inner slashed to avoid creating intermediate directories
          name, ext = relname.replace('/', '-').rsplit('.', 1)
          newname = '%s_%s.%s' % (name, self.calc_id, ext)
          if export_dir is None:
              export_dir = self.export_dir
          return os.path.join(export_dir, newname)
docs: Return the path of the exported file by adding the export_dir in
          front, the calculation ID at the end.
  
          :param relname: relative file name
          :param export_dir: export directory (if None use .export_dir)