Restructure Exctinction calculations about specutils

[kbarbary]

We should have a similar interface (which the functions have), but not the same as they do need different input parameters.

Can you clarify what you mean by "similar, but not the same"? I think you mean, "as similar as possible, but some functions will need different keywords"?

I believe that we should give the functions more meaningful names and make them none-private.

My plan was to make a public function for each model:

extinction_ccm89()
extinction_od94()
[etc]

The different ccm89-like coefficients should also not be stored in the functions itself (IMHO)

Currently they are in module-level variables, not in the functions themselves. See lines 307-314. (unless I'm mistaken what you mean here?)

[wkerzendorf]

taken care of by #55