ISO19139 to ISO19115-3 / gmd:parentIdentifier could be transformed to mdb:parentMetadata with uuidref attribute or a CI_Citation.

By default, the CI_Citation element is created.
Merge pull request #9 from fxprunayre/improvements/set-parent-identifier-strategy

ISO19139 to ISO19115-3 / gmd:parentIdentifier could be transformed using a uuiref