Use a DatabaseEntryValueObject for dataset accession

[arteymix]

This would constitute a breaking change, so I cannot introduce this in the 1.28. We already use that VO for BioAssayValueObject, so there's no reason not to be consistent for datasets.

• redefine externalDatabase and externalUri as those would become accessible from accession.externalDatabase.{name,uri}, and keep them for backward compatibility (to be marked as deprecated)

[arteymix]

This model would be a great way of exposing platform versions/update metadata we want to introduce in #378.