generic function to convert an isotope system that is part of a data frame and stitch it back together with the columns in the proper positions. uses a callback function that has to do the conversion of the isotope values

convert_isosys(iso, class_isosys, conv_fun)

## Arguments

iso - the isotope system object - the class of the isotope system to convert to - the function which converts the isotope value objects of the data frame, has to accept one parameter that is a data.frame of only the isotope value objects