AgSys crop model integration: Couple time-varying min, max and critical C:N ratios

The AgSys crop model allows for time-varying min, max and critical C:N ratios of each crop pool. It is not immediately apparent how to hook these time-varying C:N ratios into the current CTSM code, particularly code related to FlexibleCN and FUN: currently those modules use time-constant parameters defined on the parameter file for each PFT. Although it wouldn't be hard to change the code to use a time-varying value (I started to do so in https://github.com/billsacks/ctsm/commit/51241e8c3e5236ba0c0cb6adde78398ae4cd8dd2 but then reverted that commit), there are some scientific questions about how to do this correctly. Specifically, it seems like some parts of the code want a target C:N ratio for new growth, whereas other parts of the code want (for example) a target C:N ratio for all leaf growth to date, for comparison against the current actual C:N ratios of the leaves – e.g., this code in the FUN module: `delta_CN = (leafc(p)+leafc_storage(p))/(leafn(p)+leafn_storage(p)) - leafcn(ivt(p))`:

https://github.com/ESCOMP/CTSM/blob/449345ee9a534316cba0d6b4fd696b8d57e60428/src/biogeochem/CNFUNMod.F90#L1226-L1270

I'm not sure how to handle uses like that if target C:N ratios can change in time.

Our plan (from discussions with @danicalombardozzi and @pengbinpeluo) is to not try to couple the time-varying C:N ratios for now, but we will revisit this later.