Implement an influenza model

[kilianvolmer]

Motivation / Current Behaviour

We want to add support for multiple infectious diseases.

Enhancement description

Implement a model specifically for influenza infections.

Additional context

It could follow e.g. https://www.medrxiv.org/content/10.1101/2025.03.27.25324818v1

Checklist

• [x] Attached labels, especially loc:: or model:: labels.
• [x] Linked to project