Extinction setup in metal small needs updating

[karllark]

The extinction curve relationship in the metal_small example in beast-examples needs updating to be:

extLaw = extinction.Generalized_RvFALaw(ALaw=extinction.Generalized_DustExt(curve='F19'), BLaw=extinction.Generalized_DustExt(curve='G03_SMCBar'))

As this is the recommended mixture model. Basically an update of the Gordon16 mixture model to use the F19 instead of F99 Milky Way Rv relationship. F19 is better as it is based on spectroscopy in the optical instead of photometric bands.

This will change the regression tests.