same as bn13 but allowing density to exceed 1.e+18. This represents some serious approximations, physically: there are various quantities which are tabulated vs. density, and those grids (still) end at 1018. For example, the recombination into high n levels for each ion is lumped into one rate, for levels beyond those which are treated spectroscopically. Computationally, the recombination rate into these levels can be written n_e * alpha_highn(n_e, T). So the n_e which is the argument of the alpha_highn function still can't go beyond 1018. But the n_e multiplier can be arbitrarily large. Similar comments apply to some other types of rates.