Correct heavy isotope mass tendency equations

Stewart (1975) used an empirical exponent in the mass tendency equation of heavy isotopes in order to account for the turbulence occurring around a falling hydrometeor, which reduces the kinetic fractionation compared to a hydrometeor in calm air.

Contrary to Stewart (1975), we included correction of the mass ventilation coefficient in this equation (f_v'/f_v). This is not consistent with Stewart (1975), as this correction is accounted for in his empirical exponent. As (f_v'/f_v) is always positive, this consequently led to a slight overestimation of the fractionation in our isotope model.

This fix removes the factor f_v'/f_v from the heavy isotope mass tendency equations.