Implement the practical fit formula for ionization rate coefficients by Voronov (1997)

[namurphy]

The paper by Voronov (1997) includes practical fit formulae for ionization rate coefficients of atoms and ions by electron impact for hydrogen through nickel (e.g., for atomic numbers 1–28). The main benefit of these formulae is that the ionization rates can be calculated efficiently without needing a full database. The rates are accurate to a few percentage points to the best known data at the time, which is good since errors on atomic data are usually quite a bit larger than that.

The fit parameters are given in Table 1 of Voronov (1997). Fortunately, there are programs that can extract tables from pdf files, like camelot. This would be preferable to typing in the data directly since it'll be a lot less work and typos won't sneak in.