knit: clean up sml model persistence in workflow
cpp: regenerate latest model
agile: backlog xml serialisation for interoperability
agile: backlog create top-level initialisers
agile: update code generation of registrars for static registration
knit: hook it up with backend model

- implement backend interface in cpp model
- register cpp backend in backend model
- improve logging around backends
- initialise reference data in general settings factory
- remove static initialiser from formatters and fix fallout