code: def save(self, filename=None):
          
          content = self.data.yaml()
          with open(Config.path_expand(ConfigDict.filename), 'w') as f:
              f.write(content)
docs: saves the configuration in the given filename,
          if it is none the filename at load time is used.
          :param filename: the file name
          :type filename: string
          :return: