Add series editor(s)

[ja573]

We would like to add information about the editor(s) of a series.

There are two options:

• we reuse the existing contribution + affiliation schema, adding the series_id to contribution and constraining the types (e.g. can only add EDITOR as the type if there is a series_id, there can only be either a work_id or a series_id but not both)
• we recreate the structure, adding a series_editor object with similar fields to contribution, and a series_editor_affiliation with similar fields to affiliation

[ja573]

I guess it'd help clarifying what type of contributions we expect a series may have

[ja573]

We should also discuss whether we need a flag to distinguish active and historical editors in long-running series

[vincentwj]

I verge to option 1, since it's easy and I think we only need active editors (series have not run long enough to have "historical" editors). Historical editors may be an edge case?

[vincentwj]

I would say series only have editors.

[tosteiner]

Check compatibility with other standards (e.g. ONIX) to see if this translates well