docs: def url(self, filetype, base_dir=None, sasdir='sas', **kwargs):
          """Return the url of a given type of file.
  
          Parameters
          ----------
          filetype : str
              File type parameter.
  
          Returns
          -------
          full : str
              The sas url to the file.
          """
  
          location = self.location(filetype, **kwargs)
          return join(self.remote_base, sasdir, location) if self.remote_base and location else None