Ion::kappa(): changed recombination opacity to explicit LTE form

In the general form (which I took from Hauschildt & Baron (2014)), sometimes
you can end up with negative opacity, which is bad. I don't know why that
happens, but until I figure it out, I'll just use the LTE form, which is in
Rutten (2003) and Mihalas (1978).
Ray::print_ray_data(): added wavelength-integrated Planck function

This is useful to compare to wavelength-integrated J.