allow major isotope fractional abundance in an isotope system

[sebkopf]

allow having the major isotope fractional abundance listed in an isotope system but make sure it is treated approriately
--> if present, this changes the abundance to ratio conversion!
--> also think about what might happen with the rowsum ! > 1 validation constraints in case machine error is so close to 1 (but still above) if all fractional abundance including major isotope is specified
--> should there be a constraint that imposes if the major isotope is specified, all columns MUST add up to 1?
--> perhaps instead allow (include_major = F/T) as a parameter in the abundance() function which will then automatically add this one after the isotope system is initialized (and can be added again easily if an isotope system converted from say ratio is backconverted to abundance, by recasting with the abundance(..., include_major = F/T) function)