Code to replicate IDL ccm_unred

[karllark]

Would be good to have such code to replicate the functionality of ccm_unred (and fm_unred) as some general users are likely familiar with this way of using dust extinction curve.

Likely would be good to have it a bit more general where the extinction model could be picked as part of the call. Otherwise, would need one such convenience function for each extinction model. But maybe this would be good (as well as the general unred function).