Move input unit conversion and bounds checking to base class

[lpsinger]

There are two goals of this PR:

1. Self-document what units these models expect by advertising the models in standard astropy.modeling.Model metadata.
2. Eliminate input preparation code that was previously duplicated in every dust extinction model subclass.

[karllark]

Please add a description to this PR to describe in words the goal. Thanks.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.66%. Comparing base (d2e54ba) to head (4b977da). Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
dust_extinction/baseclasses.py	90.47%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage   99.89%   99.66%   -0.23%     
==========================================
  Files           9        9              
  Lines         938      900      -38     
==========================================
- Hits          937      897      -40     
- Misses          1        3       +2     

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[lpsinger]

Please add a description to this PR to describe in words the goal. Thanks.

Done.