fix variable names related to SOM in biome_defaults

[teixeirak]

In biome_defaults.csv, 3 variables need to be renamed (proportional release of GHGs for each unit soil C decomposed:
Ed_CO2_litter --> Ed_CO2_SOC
Ed_CH4_litter --> Ed_CH4_SOC
Ed_N2O_litter --> Ed_N2O_SOC

I'm not changing at the moment because that might break R code.