saving fit parameters as tables in extensions

[karllark]

Partially addresses #77

[codecov]

Codecov Report

Attention: Patch coverage is 29.16667% with 17 lines in your changes missing coverage. Please review.

Project coverage is 51.23%. Comparing base (32c78f4) to head (ace13f7).

Files with missing lines	Patch %	Lines
measure_extinction/extdata.py	29.16%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   50.69%   51.23%   +0.54%     
==========================================
  Files           6        6              
  Lines        1876     1864      -12     
==========================================
+ Hits          951      955       +4     
+ Misses        925      909      -16     

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