SIP180 Add N gain from fixation

[mswilburn]

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• src/sipnet/sipnet.c

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[dlebauer]

@mswilburn I've merged the new N-fixation scheme from #193; you can find it in the N-fixation and N-Demand sections https://pecanproject.github.io/sipnet/model-structure/#fraknitrogen-fixation-fn_textfix

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Logically I think N fixation needs to be handled before N limitation, i.e. the sequence should be:

• calculate N demand
• calculate N_fix
• re-calculate N demand
• calculate N update
• calculate N limitation & apply effects on growth

N fixation is designed to satisfy plant N demand - it adds directly to plant N and doesn't add to the N min pool (the next level of complexity would be to represent free-living n-fixers by allowing some or all fixed n to be added to N_min.)