Add N gain from fixation

[Alomir]

Implement F^N_fix

... (update: see formulation in #193) ...

• add two new params
• Add F^N_fix to N_min
• Write F^N_fix to sipnet.out
• Write unit test for F_N_fix
• Add new parameters to russell_2 smoke test and verify new results make sense
• Update CHANGLELOG.md

For the unit test, it is time to revamp testNitrogenCycle.c to be less hacky (initState() split up, checkFlux either split up or made one proper comparison function)