Add SEIR Epidemic Model with Vaccination Policy

While exploring the existing examples, I noticed there's no population-level epidemic spreading model. The `virus_antibody` example is great but it works at the biological/cellular level ,there's nothing showing how a disease spreads through a population on a grid and how interventions like vaccination affect that.

I'd like to propose a SEIR model (Susceptible → Exposed → Infected → Recovered) where a government meta-agent monitors infection rates and triggers vaccination campaigns when a threshold is crossed. The idea is similar to the hierarchical activation pattern , individual agents spreading infection bottom-up, and a policy agent responding top-down.

I think it would complement the existing examples well and demonstrate Mesa 3.x grid interaction and meta-agent patterns in a context most people are already familiar with.

Happy to implement this if it fits the direction of the repo. Let me know if you'd like any changes to the scope. @jackiekazil @quaquel @catherinedevlin @rht @dmasad

Start by reviewing the existing virus_antibody example and the repository's Mesa 3.x grid interaction and meta-agent patterns. Done would be a population-level SEIR example in which individual agents spread infection and a government meta-agent triggers vaccination campaigns after infection rates cross a threshold.

Written by the indexing model from the issue text.