Review temperature-dependent biological nitrogen fixation rates

@hrlai is already working on this.

Since Feb 17, 2025.

Currently, biological nitrogen fixation in the soil module follows [Lin et al. (2000)](https://doi.org/10.1016/S0304-3800(00)00372-0), who used a temperature-dependent cosine function for symbiotic fixation

$$
f_\text{SYN} = \cos \left( \dfrac{T - T_\text{opt}}{T_\text{max} - T_\text{min}} \right)
$$

and a Q10 function for non-symbiont

$$
f_\text{NSYN} = Q_{10}^{\frac{T - T_\text{opt}}{10}}
$$

This study is rather dated so I am going to look for a more updated model. As a start, the Lin cosine function is a curve-fitting that ensures the maximum to be 1. Then for $T$ below -5°C or above 60°C the truncated $f_\text{SYN}$ to be zero (so the function is non-negative). The peak is at $T_\text{opt} = 20$, which is around what recent literature uses. As a first pass I will review if such a unimodal relationship is still warranted, and then find out whether the functional form / parameters needs to be updated.