Lookup the values available for querying.

def catalog(self, table='', column=''):
        """Lookup the values available for querying."""
        lookup_table = self.lookup_table
        if lookup_table is not None:
            if table:
                if column:
                    column = column.upper()
                    return lookup_table[table][column]
                return lookup_table[table]
            # Show what methods are available.
            return self.lookup_methods
        return None