I'm not familiar enough with the FM function. What exactly are the differences between f99 and fm07? I ask because if it is really a completely different model, separate functions might be more appropriate.

If in CCM the keyword argument is simply changing coefficients but it is doing something else in FM, it might be worth breaking the symmetry.

We might also want to consider uniting these functions in a single `extinction` function. If we do that, then `model` might be more appropriate name (since the models for CCM and FM are completely different).