Dust-extinction

================

- Added D22_MWAvg average and D22 parameter average models

================

- more NIR/MIR average models added (RRP89, G21_MWAvg)
- NIR/MIR shape model (G21)
- dust grain models added (WD01, D03, J13, ZDA04)

================

- 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

================

- VCG04, GCC09, FM19 models added

================

- 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)

================

- minor updates to release version, notes