Clarify when use_c13/use_c14 and carbon isotopes timeseries can be used

[ekluzek]

build-namelist doesn't restrict when use_c13/use_c14 can be turned on. It does restrict use_c13_timeseries and use_c14_bombspike to be only on when use_c13 and/or use_c14 are on respectively.

The code doesn't allow use_c13 or use_c14 to be on when FATES is on.

However, both soilbiogeochem and biogeochem use use_c13/use_c14, so can use_c13/use_c14 be allowed when use_cn is NOT on?

In controlMod you can always set them -- but they only get printed out when use_cn is true.

CNVegetationFacade has some use_c13/use_c14 uses inside if statements of either use_cn or use_fates_bgc...

if(use_cn.or.use_fates_bgc)then

dynSubgridDriverMod calls DynamicAreaConservation from CNVegetationFacade which has use_c13/c14 if statements from inside a use_cn if statement. CNDriver has uses in CNDriverNoLeaching, CNDriverSummerizesStates, CNDriverSummerizesFluxes. And these are called from clm_driver, with some in use_cn if statements and some as above...

       if (use_cn .or. use_fates_bgc) then

[ekluzek]

I think this means that use_c13/use_c14 can only be on when use_cn is on, as that would be the closest to what the code is doing now. If that's the case, we should clarify this further in the namelist handling in both the buildnml level and in the Fortran code level.