Events from Crossref will only include data citations

In order to improve stability of Event Data while continuing our support for data citations, we are making some changes to the Crossref source in Event Data. We have been collecting all relationships and references. This is a large amount of data and we have had difficulty processing it all, resulting in a large number of missed events.

The most important use case that we are supporting with this source is identifying data citations made by Crossref works, which includes referencing DataCite DOIs and references to datasets with a Crossref DOI. We have therefore decided to focus only collecting these events rather than a comprehensive set of references and relationships. This is a much smaller set of events and we expect to be able to process them much more effectively. All of the relationship and reference metadata will remain available via our primary REST API.