Add in D03 grain models

[karllark]

Adding in the first set of extinction curves based on dust grain models.
Useful for comparisons between such models and observations.
Also provides a physics based extrapolation of extinction to non-observable wavelengths (e.g., below 912 A).

Closes #6.

[codecov-commenter]

Codecov Report

Merging #167 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #167   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         8    +1     
  Lines          554       588   +34     
=========================================
+ Hits           554       588   +34     

Impacted Files	Coverage Δ
dust_extinction/grain_models.py	100.00% <100.00%> (ø)

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