PUBLIC function: calculates the Henry's constant (solubility) for O2
salinity in practical salinity units (i.e. no unit)
temperature in degrees centigrade
the Henry's constant for CO2 in mol/(kg-solution*atm)
Andreas F. Hofmann. Maintained by Karline Soetaert (Karline.Soetaert@nioz.nl).
derived from a formulation for [O2]sat given in Weiss1970