isotopic abundance corrections mess up default radiative gamma recipe

When the radiative broadening factor is not available, it is estimated from the wavelength and `log_gf` value. That `log_gf` value will be wrong if it was adjusted for isotopic abundances.

#173 addresses this for turbospectrum and VALD linelists in the case that Korg does the adjustment itself, but in cases where VALD has already done the adjustment, it's not obvious what should be done.

options:
- Nothing, just add a warning to the documentation. (I don't like this one.)
- Refuse to parse. This is easy and prevents things from going wrong, but it would likely annoy people.
- Try to do things correctly. We could in principle undo the adjustment if we knew what VALD assumed, and it will work as long as they never change it. This is the most seemless, but it's also brittle.