code: def get_documents(self, subtypes=None, refresh=False):
          
          search = ScopusSearch('au-id({})'.format(self.identifier), refresh)
          if subtypes:
              return [p for p in search.results if p.subtype in subtypes]
          else:
              return search.results
docs: Return list of author's publications using ScopusSearch, which
          fit a specified set of document subtypes.