Update documentation in sipnet/docs/model-structure.md to reflect actual implementation of N pools and fluxes.

• Change tense ('will be'; 'will represent' --> represents)
• Move Justifications below description (see David's comment from Add N loss to volatilization #89 for specifics)
• Define K_vol as, perhaps, "Nitrogen volatilization rate constant that determines the maximum rate of N volatilization as a proportion of available N_min."
• Change N leaching flux to new equation NLeachingFlux = minN * phi * fNLeach where phi = min(drainage/soilWHC)
• Remove separation of litterMinN pool and soilMinN pool to reflect implementation of single minN pool

Update documentation in sipnet/docs/parameters.md to reflect implemented N cycle parameters

• Add K_vol to parameters.md
• Fix redundancy in Nitrogen Cycle Parameters section with K_vol and fN2O_vol (we only implemented nVolFrac, not sure what the second parameter listed is?)
• Add fNLeach as parameter