Implement SEI-type models other than SEIRD or SVEIRD

[bryce-carson]

Epidemic Model
🔘 SEIRD
🔘 SVEIRD

In the old application and its associated SVEIRD model implementation a user was able to enable or disable vaccination. It would be nice and interesting if we could extend spatialEpisim to other SEI-type models by writing a higher-order function to handle enablement or disabling of various compartments.