Add graceful fails to prevent P-coupling with CLM

[rgknox]

In FATES we have an option when running in nutrient enabled (ie CNP) mode, that allows the user to prescribe the N or P uptake rates, or accept the uptake rates detrmined by the competition scheme in the land model. This is controlled in the parameter file, with fates_cnp_prescribed_nuptake and fates_cnp_prescribed_puptake

Values of 0 signal there should be coupling, where positive values signal there should not be coupling and FATES should generate uptake synthetically. Why do we have prescribed rates? Firstly, it allows us to test FATES-CNP, by removing variability in coupled uptake. And secondly, it allows us to simulate the limitation of one chemical species instead of both.

In most cases, when using prescribed uptake, the value should be set large, ie 1 or greater, so that chemical species has no chance of limiting the simulation. Any excess will simply be lost to litter fluxes.

CLM does not cycle phosphorus, and thus FATES cannot exchange phosphorus with CLM. So CLM should never be run with P coupling active.

We should add protections in the code, here, so that the model fails if the host model is CLM and p_uptake_mode= coupled_p_uptake.