adding option for FM90_B3 to FM99_method

[karllark]

Useful for measure_extinction.modeldata. Allows fitting for the amplitude of the 2175 A bump (B3) instead of the "strength" C3.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.55%. Comparing base (9b47a49) to head (bc6e1bf).

Files with missing lines	Patch %	Lines
dust_extinction/shapes.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
- Coverage   99.66%   99.55%   -0.12%     
==========================================
  Files           9        9              
  Lines         899      901       +2     
==========================================
+ Hits          896      897       +1     
- Misses          3        4       +1     

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