code: def mfw_(self, col, sw_lang="english", limit=100):
          
          df = self._mfw(col, sw_lang, limit)
          if df is None:
              self.err("Can not find most frequent words")
              return
          return self._duplicate_(df)
docs: Returns a Dataswim instance with the most frequent words in a
          column exluding the most common stop words