Before this commit, any value could be added to registries and the class
needed to be overridden to assert that added values would answer to a
certain predicate.
Now, the registry can be instantiated with a given predicate function
that all added value must pass to be registered (default: all values
are accepted).
Existing implentations have been adapted:
- FieldRegistry
- FieldRegistryOwl.
Julien Mougenot
5698d1fab9