Add WD Hubeny models

[karllark]

Adding Hubeny WD model grid. Useful for absolute flux calibration work and, possibly, measuring local dust extinction at low dust columns.

Also fix a bug in reading in the Spitzer LRS model spectra - Jy units not correct. May cause issue with real data - will fix when that happens.
Also fix a doc bug on astropy dev docs.

[codecov]

Codecov Report

Attention: Patch coverage is 20.83333% with 38 lines in your changes missing coverage. Please review.

Project coverage is 39.80%. Comparing base (706d4e6) to head (ac6280e).
Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
...easure_extinction/utils/make_all_tlusty_obsdata.py	7.40%	25 Missing ⚠️
measure_extinction/stardata.py	38.88%	11 Missing ⚠️
measure_extinction/utils/fit_model.py	0.00%	1 Missing ⚠️
measure_extinction/utils/merge_stis_spec.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   40.10%   39.80%   -0.31%     
==========================================
  Files          22       22              
  Lines        2977     3007      +30     
==========================================
+ Hits         1194     1197       +3     
- Misses       1783     1810      +27     

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