Toml dict is missing `R_d`

[charleskawczynski]

The toml dict is missing R_d, and we need to add it. It's used in RRTMGP.jl. What is a derived parameter for one package (R_d for Thermodynamics) may not be derived for another package (RRTMGP.jl).

[charleskawczynski]

I suppose that, in this case, we can massage the arguments so that RRTMGP.jl stores gas_constant and molmass_dryair in order to define a derived parameter method.