Add NHP model

[tizianocitro]

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Description

• Added NHP model.
• Added NHP example.

Checklist

• Does your submission pass all tests? (use make test)
• Have you written tests to cover all your changes? If not, provide a reason.
• Have you lint your code locally before submission? (use make lint)
• Have you type checked your code locally before submission? (use make typecheck)
• Have you added an explanation of what your changes are and why you'd like us to include them?