format_nucl() formats chemical isotopes in nuclear
notation.formatdown_options() gains the Z and
warn arguments used by format_nucl().format_numbers(0, format = "sci") now correctly yields
a zero without power-of-ten notation. Likewise for
format = "engr" (#10).formatdown_options(), multiply_mark can
now be set to "\\cdot" (center dot), often used when
decimal_mark is a comma.