updates sub_total counts for the class instance based on the
            current dictionary counts

        args:
        -----
            current: current dictionary counts
            sub_key: the key/value to use for the subtotals

def update_subtotals(self, current, sub_key):
        """
        updates sub_total counts for the class instance based on the
            current dictionary counts

        args:
        -----
            current: current dictionary counts
            sub_key: the key/value to use for the subtotals
        """

        if not self.sub_counts.get(sub_key):
            self.sub_counts[sub_key] = {}
        for item in current:
            try:
                self.sub_counts[sub_key][item] += 1
            except KeyError:
                self.sub_counts[sub_key][item] = 1