New gotm_n2o.yaml file

[wathen]

Need to add new gotm_n2o.yaml to run the the n2o fabm yaml files in ERSEM

partial_pressure_of_n2o:                             # partial_pressure_of_n2o, https://gml.noaa.gov/aftp/data/hats/n2o/combined/GML_global_N2O.txt [n atm]
         method: constant                            # method [constant, file=from file; default=constant]
         constant_value: 338.87                      # value to use throughout the simulation [n atm; default=0.0]
         file:                                       # path to file with time series [default=]
         column: 1                                   # index of column to read from [default=1]
         scale_factor: 1.0                           # scale factor to be applied to values read from file [default=1.0]
         offset: 0.0                                 # offset to be added to values read from file [Pa; default=0.0]