docs: def notify_progress(self):
          ''' only call the progress callback if total has changed
              or PROGRESS_MSGS_SEND_ALL is set '''
          _total = 0
          for _session in self._sessions.values():
              _total += _session.bytes_transferred
  
          if AsperaSession.PROGRESS_MSGS_SEND_ALL:
              self._run_progress_callbacks(_total)
          else:
              # dont call progress callback unless total has changed
              if self._total_bytes_transferred != _total:
                  self._total_bytes_transferred = _total
                  self._run_progress_callbacks(_total)