Dust-extinction

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- NIR/MIR average models added
(RL85_MWGC, I05_MWAvg, CT06_MWLoc, CT06_MWGC, F11_MWGC)
- updates to be remove astropy 4.0 deprecated modeling features
- Documentation updates
- Many tests consolidated
- Bugfixes

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- VCG04, GCC09, FM19 models added

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- GCC09_MWAvg, O94, F04, M14 models added
- AxAvtoExv helper model added
- significant updates to the documentation
- extinction versus attenuation documentation added
- models reorganized into 3 categories (*API change*)
- made a new base class for R(V)_A, f_A dependent models (G16, expecting more)

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- minor updates to release version, notes

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- initial release