Implement n-year survival probability function

[ShannonT20]

Description

Implement the n-year survival probability function for the mortality module.

Expected formula:

nPx = Product of survival probabilities from age x to x+n.

Tasks

• Add npx(age, n) method
• Add unit tests using pytest
• Validate outputs using sample mortality tables

Technologies

• Python
• pytest
• NumPy

Benefits

What benefits would this feature provide to:

• Users of ACTUNEO
• The African actuarial community
• The broader actuarial profession

Risks and Considerations

Potential risks, drawbacks, or considerations:

• Performance implications
• Complexity increase
• Maintenance burden
• Backward compatibility

African Context

How does this feature specifically benefit African actuaries or address African actuarial challenges?

Related Issues/PRs

Link to any related issues or pull requests.

Additional Context

Any other information that might be helpful for understanding and implementing this feature.