Prepare for tallying. Create a new chain.

def _initialize(self, chain, length):
        """Prepare for tallying. Create a new chain."""
        # If this db was loaded from the disk, it may not have its
        # tallied step methods' getfuncs yet.
        if self._getfunc is None:
            self._getfunc = self.db.model._funs_to_tally[self.name]