adding new isotope equil. fractionation factor formulae for ice-vap 2H (LambEtAl2017, EllehojEtAl2013) + new example notebook (Fig. 4 from Lamb et al. 2017, PNAS)

adding new isotope equil. fractionation factor formulae for ice-vap 2H (LambEtAl2017, EllehojEtAl2013) + new example notebook (Fig. 4 from Lamb et al. 2017, PNAS) #1108