code: def summarize_dataframe(self):
          
          if self.dataframe_hash:
              return(self.dataframe_hash)
          else:
              df = self._as_dataframe_unmodified()
              return(self.dataframe_hash)
docs: Summarize default dataframe for this cohort using a hash function.
          Useful for confirming the version of data used in various reports, e.g. ipynbs