G24_SMCAvg model interpolation/normalization

[petiay]

The G24_SMCAvg extinction curve model may benefit from a smoother interpolation between data points. This becomes apparent when comparing it with, say, the G23 (Rv=3.1) model where I would expect the curves would intersect at the V-band wavelength. The second plot shows the G23 (Rv=2.74) curve together with the G23 (Rv=3.1) model for comparison.

[karllark]

A good idea. This was done for the G03_SMCBar average and clearly something similar is needed here.