sero to-do list

[AugustHao]

• make a sero-specific version of observation model function
• make version of sero obs model without reinfection submodel
• move dynamic model bits into a separate dev branch AFTER making prototype sero model w/o reinfection work
• generate dummy sero data to test with
• merge sero-specific observation model function into the main function with arguments to control if sero or case version should be used