Fix SVJ model

[lsbardel]

• Add more docs
• Add black_delta function
• Fix double exponential distribution
• Fix Heston with exponential Jumps
• Add exponential sampling example

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.72727% with 24 lines in your changes missing coverage. Please review.

Project coverage is 67.73%. Comparing base (6a4ee9a) to head (c6b9858).

Files with missing lines	Patch %	Lines
quantflow/options/calibration.py	46.66%	16 Missing ⚠️
quantflow/options/bs.py	20.00%	4 Missing ⚠️
quantflow/utils/distributions.py	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   68.21%   67.73%   -0.49%     
==========================================
  Files          41       41              
  Lines        2495     2526      +31     
==========================================
+ Hits         1702     1711       +9     
- Misses        793      815      +22     

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