sets `self.number_chars` to the number of characters as string.

def _extract_total_number_of_chars(self):
        """
        sets `self.number_chars` to the number of characters as string.
        """
        self._get_gene_codes_and_seq_lengths()

        sum = 0
        for seq_length in self._gene_codes_and_lengths.values():
            sum += sorted(seq_length, reverse=True)[0]
        self.number_chars = str(sum)