Add SMC Bar dust to the dust model

[karllark]

The full range of know dust extinction is significantly larger than just the R(V) dependent Milky Way relationship (e.g. CCM89). The dust seen in the Magellanic Clouds deviates strongly (see Gordon et al. (2003)). A 2nd parameter that would be fraction of SMC Bar extinction, f(SMC), should capture the full, observed range of dust extinction curves. This would enhance the dust model to have 3 parameters - A(V), R(V), & f(SMC).