EDIT

In #6581 we replaced reference links by sources (e.g. for SingleSourcedEntityBase). Classification is already IdentifiableEntity/SourcedEntityBase and therefore has already multiple sources. So we need to discuss how to unify these two possibilities to add source(s) to a classification.

Probably we have to move the existing source to the set of sources and remove the possibility to define a single source.

Problematic in this solution is that root taxa inherit their sec reference from classification by default. Maybe we could use the only PrimaryTaxonomicSource automatically if there is not more than 1, otherwise we need user feedback.

Also we need to discuss that we have a similar situation for taxa where we have the sec source and other sources, but this is slightly different as sec is meant for defining the concept.

For this we need a possibility in TaxEditor to edit the sources for Classifications which is currently not possible.

Note: As the single source is also used in TaxEditor to compute the secundum sources for top-level taxa an explicitly single source does seem to make sense (for now) and therefore we will implement it.