F99 and FM07 are not completely different - both are OIR spline + Drude function for 2175A bump + FUV rise. They just have slightly different functional forms (they share most, but not all, of their parameters). (I may just drop FM07 entirely if I can't get in contact with Fitzpatrick... FM07 doesn't appear to fully specify the R-dependent behavior.)

It's not a bad idea to make them a single function, but I think for the moment it's preferable to keep them separate - they are two very different ways to doing it, and the current `model` parameter does relatively minor tweaks. Also, it's possible to have a GCC09 version of FM, which would be entirely different from the GCC09 version of CCM. (I would love for this to be the case - the data's there, but GCC09 didn't bother to get the proper R dependence for the FM parametrization.)