update build namelist logic checks on fates and AD

Work to get the N-cycling version of FATES coupled with CLM is in the works. The current coupling is c-only, and therefore there is no N uptake by plants, or deposition of N in fates generated litter. In c-only fates mode, accelerated decomposition seems somewhat meaningless, because the below-ground decomposition dynamics cannot be limited by nutrient availability in any meaningful way.

@ekluzek and I were looking at the namelist logic and thought it would be wise to have a graceful fail, if there is agreement that FATES in c-only mode with AD is a misleading thing that should not be done.

Here is the setup_cmdl_spinup routine: https://github.com/ESCOMP/CTSM/blob/ctsm5.1.dev127/bld/CLMBuildNamelist.pm#L1116

We could potentially add an extra if clause for a FATES case before the else here:

https://github.com/ESCOMP/CTSM/blob/ctsm5.1.dev127/bld/CLMBuildNamelist.pm#L1156