feat(pricing): add black-76 pricing model

[carlobortolan]

Closes #40.

CC #15.

[codecov]

Codecov Report

Attention: Patch coverage is 95.76720% with 8 lines in your changes missing coverage. Please review.

Project coverage is 93.42%. Comparing base (63e0d83) to head (eaef921).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/options/models/black_76.rs	96.15%	6 Missing ⚠️
src/options/models/black_scholes.rs	96.42%	1 Missing ⚠️
src/options/models/monte_carlo.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   92.86%   93.42%   +0.55%     
==========================================
  Files          19       19              
  Lines        2215     2356     +141     
==========================================
+ Hits         2057     2201     +144     
+ Misses        158      155       -3     

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