Increasing "watersat" flexibility

[jnaar]

"watersat" subroutine computes the theoretical "qsat" which is q(igcm_h2o_vap) in kg/kg. To do so, it uses Goff-Gratch equation to compute Psat at a given temperature, then divides it by P and convert it from vmr to mmr using a 0.41 factor, which is actually 18/44 (M_h2o / M_atm). Under specific conditions, the molar mass of atmosphere may be quite different from M_co2. Adapt watersat_mod.F to allow for inputing atmospheric molar mass.