02-03-13 Gas Flux model

First preliminary introduction of Gas flux model. New chapter still not documented nor tested.

Deleted bug on Dissolved organics, error in previous initial values.

Introduced litter fall from grains at harvest.

Made emergence to the day of root development. This means that roots, stem and leaf start to develop at this date.

Corrected calculations of C-N ratios for perennial plants.  PEJ/LK