create an isotope value object (this function is not exported and should
be access via the appropriate wrapper functions, e.g.
iso(class_isosys, ..., attribs = list(), single_as_df = FALSE)
name of the class for an isotope system (which holds the info on which isoval class belongs to the system as well)
values (can be single data frame or list)
named list of attributes to pass to the isotope data object constructors
whether to return a single value as a data frame
the setup for this function also means that you can modify e.g. an existing ratio with the paramters passed in (say to set the name later on)