add Repository with a well known global interface

- add Repository with a well known global interface
    - see [RKazam](https://github.com/r-dbi/RKazam) for DBI interface
    - wrap Dictionary in DBI interface?
- replace [demo-service-UnitOfWork.R](https://github.com/tidylab/ddd/blob/feature/add_domain_service/R/demo-service-UnitOfWork.R) interface implementation