Constants required for calculations.
constDefaultsGSC: solar constant (0.0820 [MJ/(m2 min)])
sigma: Stefan-Boltzmann constant (4.903 10^-9 [MJ/(K4 m2 day)])
a1: constant lapse rate moist air (0.0065 [K/m]) (e.g. in estP)
R: specific gas constant (287 [J/(kg K)]) (e.g. in estP)
g: gravitational acceleration (9.807 [m/s2]) (e.g. in estP)
eps: ratio molecular weight of water vapour/dry air (0.622) (e.g. in psyc_cons)
lambda: latent heat of vaporization (2.45 [MJ/kg]) (e.g. in psyc_cons)
cp: specific heat at constant pressure (1.013 10-3 [MJ/(kg degreeC)]) (e.g. in psyc_cons)