Make natural abundances available

[tkittel]

As described very well by the usecase of @KBGrammer in #144, it would be useful for NCrystal to also provide natural abundances, to let the composition-flattening use it by default, etc.

We might also ultimately want such data anyway, to improve various models (e.g. more realistic per-isotope absorption models, free-gas models with specific rather than averaged masses, etc.).