Support for the programmatic creation of repository model instances

Full test of the API
Most of the features of the API are tested, but it's a good idea to create a larger example that uses all/most parts of the API. Possible failures are entities that cannot be fetched from the repository even though they should be added and NullPointerExceptions because of not instantiated lists. Furthermore, imports have never been tested fully.

Validate repository
It would be nice if the created repository would also be fully validated when calling the finishing method createRepositoryNow(), i.e. values of required fields are set, every SEFF has exactly one start + stop action etc.
See TODO comment in RepositoryCreator.java.