MIMICS litter allocation, fmet, is not currently applied

Currently MIMICS is running with the litter fractions defined for each PFT that are allocated to metabolic and structural litter pools (both for C and N). This happens in the [CNLitterToColumn](https://github.com/ESCOMP/CTSM/blob/c030c2342dac53d30ed419aca262db8fa7213ee6/src/biogeochem/CNPhenologyMod.F90#L3857) subroutine of `CNPhenologyMod`.

This is different than the MIMICS approach that calculates this allocation based on litter chemistry (`fmet`, or fraction of metabolic litter which is a function of lignin:N ratio of litter inputs). Note, fmet is then used in the parameterization of mimics fluxes in the BGC code too.

For now I don't think it's worth addressing now, but we may want to consider this down the road.

Also tagging @elisacw