Deprecation of "warmup"

[twallema]

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have updated the documentation accordingly.

Describe your fixes/additions/changes

What was warmup?

"warmup" was a legacy feature from the COVID-19 pandemic, it was a parameter (int/float or date) that could be given to the sim function, allowing the simulation to start prior to the start date. Its value could be calibrated to determine when an epidemic was seeded.

It has persisted until now because it didn't harm anyone.

Why remove warmup?

• PR Initial condition functions #99 can be used instead. Users wanting to use this epi-legacy-feature can just infer the initial number of infected in their epi models to find similar results.
• To keep the code clean and as easily accessible as possible.