code: def dateindex(self, col: str):
          
          df = self._dateindex(col)
          if df is None:
              self.err("Can not create date index")
              return
          self.df = df
          self.ok("Added a datetime index from column", col)
docs: Set a datetime index from a column
  
          :param col: column name where to index the date from
          :type col: str
  
          :example: ``ds.dateindex("mycol")``