Add support for "Iab" and "I" luminosity classes, as well as decimal subtypes (e.g. M4.5)

[nussun-celestia]

When making stars with certain spectral types, there is no support for "Iab" or "I" luminosity classes. "Iab" gets interpreted as "Ia", and "I" gets interpreted as "Ib". "Iab" is a commonly used luminosity class so it doesn't make sense why it is not included. "I" is usually used when the supergiant luminosity class subtype is not known, this could be useful to have too. Another useful feature would be to have support for decimals in the subtype, e.g. if you specify "M4.5" it gets interpreted as "M4" which is incorrect too.