Updated the data API to include a new type of individual: the RelativeIndividual. This individual, identified by RelativeIndividualId, is meant to represent resources identified with URIs relative to a ManagedIndividual, but which are not controlled by the LDP4j Application Engine. That is, the URIs cannot be dereferenced. It might be worth pointing out that the lifecycle of the application may conflate this individuals with proper ManagedIndividuals if attachments for the parent resources of the RelativeIndividual are created by the LDP4j Application Engine using the same relative paths.