re-evaluate prescribed nutrient uptake in uncoupled model

[rgknox]

With the forthcoming PR #598, we allow an uncoupled simulation mode, where we can prescribe nutrient uptake rates, and break the nutrient connection to the soil (if optioned in E3SM, or the exclusive option in CTSM).

Currently, the uptake rates are calculated like so:

1. assess a cohort's nutrient needs for the current daily time-step by evaluating the total nutrient needed to match NPP and get the plant tissues up to optimal CN and CP ratios in all tissues. We do this by running the allocation scheme an extra time, where we pretend there was an inordinate amount of nutrient available, and document how much was actually allocated.
2. for the next time-step, we average in the need calculated in step 1 to a smoothed daily demand. demand = a*demand + (a-1)*need
3. the actual uptake on the next step is calculated as the demand from step 2, multiplied by a user specified parameter: the fraction of the demand desired for uptake. Setting this to >1 will seemingly give the plant plenty of nutrient and prevent nutrient sink limitations to growth

Issues:

1. See comment by @ckoven here: Nutrient Enabled Aquisition (parteh pt 3) #598 (comment)
2. The calculation of the need in step 1, is inefficient, it makes us run the allocation solver twice
3. My sense is that this should have some conceptual basis, whereas right now it is just kind of using some type of circular logic

[rgknox]

Closing, this is referencing old methods of specifying prescribed uptake.