Initialize BGC profile variables to nan for FATES

[ekluzek]

The BGC variables: croot_prof_patch, froot_prof_patch, leaf_prof_patch, and stem_prof_patch have to be allocated to the full size for FATES, because they are passed to fire interfaces, but to ensure they are unused we should initialize them to nan rather than spval for FATES. Right now they are initialized to spval, and that should just be done for non-FATES and when they are declared as history variables rather than always.

[ekluzek]

Something that I noticed a long time ago, that would in principle help with things like: #2932