@stock_and_flow no longer defined in the module

The example given in the README file on seir model uses the `@stock_and_flow` macro which is not found in the modules. 

> UndefVarError: `@stock_and_flow` not defined in `Main.var"workspace#44"`
> 
> Suggestion: check for spelling errors or missing imports.

Is this an outdated method? If yes, please let me know where to find the latest one