Implement Heston model

[mvanzulli]

Closes #21

The implemenation has been validated against QuantLib:

>>> option_price_ql = option.NPV()
>>> print(f"Option Price using QuantLib: {option_price_ql:.16f}")
>>> on Price using QuantLib: 8982.8708666875791096

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (844ec7d) to head (aa10b08). Report is 11 commits behind head on master.

Files	Patch %	Lines
src/models/heston.jl	96.77%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #28 +/- ## ========================================== + Coverage 83.64% 86.55% +2.90% ========================================== Files 13 15 +2 Lines 159 238 +79 ========================================== + Hits 133 206 +73 - Misses 26 32 +6 ```

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[mvanzulli]

@iblislin this is ready for review 👼🏼

[mvanzulli]

@iblislin this is ready for review 👼🏼

@iblislin let me know if I can help with the review :)

[iblislin]

ah, sorry, I'm swamped with work recently... I don't check the algorithm detail, but I think the coding style or syntax are fine for me. let's just merge it first. :rocket:

[mvanzulli]

ah, sorry, I'm swamped with work recently...

I don't check the algorithm detail, but I think the coding style or syntax are fine for me.

let's just merge it first. :rocket:

No worries used to happen, thanks for the review 🙌🏻🫶🏼👌🏼