fix: imports in virus_antibody model

[rico-haas]

Summary

Due to a relative import, it was not possible to run the virus_antibody visualization using solara run app.py. I moved the files into a module.

Bug / Issue

When running the virus_antibody example using solara run app.py, as instructed in the README:

...
  File "/home/rico/Repositories/mesa-examples/examples/virus_antibody/model.py", line 12, in <module>
    from .agents import AntibodyAgent, VirusAgent
ImportError: attempted relative import with no known parent package

Implementation

I moved all code files except app.py into a module, like any other example using SolaraViz does it. Similar PR: #262
I also cleaned up the imports while I was at it.

Testing

No automated tests; I manually ran it in my venv and it works now.