Change preferred order of PublicationYear in oai_datacite

Created by: yytosi

Study.publication_years.attr_distribution_date.get_value() should be preferred instead of Study.publication_years.get_value() for lookup for PublicationYear value in oai_datacite serialization.

Also, the year should be extracted from either value.

This issue is related to https://bitbucket.org/cessda/cessda.cdc.aggregator.oai-pmh-repo-handler/issues/30/format-the-value-of-publicationyear-in