feat(pricing): implement black scholes model

[carlobortolan]

• Implement black scholes model
• Update code docs
• Refactor options structs into separate module to allow more exotic option styles

See #3, #5, #15.

[codecov]

Codecov Report

Attention: Patch coverage is 98.43505% with 10 lines in your changes missing coverage. Please review.

Project coverage is 98.56%. Comparing base (c529b99) to head (4f024b9).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
src/options/models/black_scholes.rs	98.03%	6 Missing ⚠️
src/options/types.rs	0.00%	3 Missing ⚠️
src/options/models/monte_carlo.rs	99.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   99.40%   98.56%   -0.84%     
==========================================
  Files           5       11       +6     
  Lines         336      699     +363     
==========================================
+ Hits          334      689     +355     
- Misses          2       10       +8     

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