Implement N uptake and N limitation

[dlebauer]

Implement uptake and limitation as described in model_structure.md, which is repeated here:

Uptake

• Uptake = N_demand - N_fixation

Limitation

• Calculate the amount by which plant N demand exceeds available supply
• Calculate the fraction by which biomass growth must be reduced so that N demand equals supply.
• Reduce biomass growth accordingly by scaling carbon allocation to plant biomass pools.
• Calculate nitrogen uptake as the amount of N required to support the realized plant growth, based on fixed stoichiometry.
• Carbon associated with the unmet growth demand is subtracted from potential GPP to maintain mass balance

Tasks

• Add relevant fluxes needed to support this (no parameters needed, I believe)
• Add test(s) in tests/sipnet/test_modeling, possibly in testNitrogen.c
• Update docs
• Update CHANGELOG.md

[Alomir]

Uptake was implemented as part of #265

[Alomir]

Design change: we are no longer reducing GPP when N limitation happens