Change preferred order of PublicationYear in oai_datacite

[Toni Sissala]

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

[Toni Sissala]

Change preferred lookup order for oai_datacite PublicationYear

For oai_datacite PublicationYear, the Study.publication_years.attr_distribution.get_value() is primary, Study.publication_years.get_value() is secondary.

Format the value so that it should contain only a year instead of full datestamp. The formatting expects datestamp format that begins with a full year (for example YYYY-MM-DD).

Bump version to 1.0.2.

Write unreleased changelog entry for 1.0.2.

Fixes #33 at BB.

→ <<cset 3a6b0b1b74f3>>