Return new class:`PseudoTable` object with pseudos in the given rows of the periodic table.
        rows can be either a int or a list of integers.

def select_rows(self, rows):
        """
        Return new class:`PseudoTable` object with pseudos in the given rows of the periodic table.
        rows can be either a int or a list of integers.
        """
        if not isinstance(rows, (list, tuple)): rows = [rows]
        return self.__class__([p for p in self if p.element.row in rows])