Fix carbon isotope bug that caused wrong answers for isotopes under transient land-use change

We currently have a bug so that for transient land-use change cases answers are different starting in soil
carbon when Carbon isotopes (use_c13, or use_c14) are on versus off. Answers are identical if there is no
land-use change. The bug does cause bulk Carbon (C12) to be slightly different, but qualitatively the same.
There is a significant impact to the Carbon isotope simulation however. The bug was causing changes in isotopic
pools to be directed to the bulk Carbon. Because, isotopes are so much smaller than bulk Carbon, this doesn't
cause a marked difference in the bulk Carbon simulation. But, the lack of the change in the isotopic pool
does have a meaningful impact on the simulation of the Carbon isotopic fields.