code: def add_file_arg(self, filename):
      
      self.__arguments.append(filename)
      if filename not in self.__input_files:
        self.__input_files.append(filename)
docs: Add a file argument to the executable. Arguments are appended after any
      options and their order is guaranteed. Also adds the file name to the
      list of required input data for this job.
      @param filename: file to add as argument.