Check for existence of related object in IngestedRelationshipsTransformer

Added logic and configuration options to control the behaviour of the IngestedRelationshipsTransformer's handling of related objects.

1) Check for existence of the related object before creating the relationship. This logic is activated by using the boolean option "checkExistence" (which may exist within the relationship definition of the JSON config).

2) Forcibly update relationship metadata (at harvest time) - overriding the existing PROPERTY_FLAG check. This logic is activated by using the boolean option "forceUpdate" (which may exist directly under the "ingest-relations" section in the JSON config).