Referencenumber formatter: configurable `dossier_document_seperator`

I implemented the dossier_document_seperator wich separtes the dossier and the document part in the referenceNumber configurable.

This is used in the `grouped_by_three` formatter, which use hyphen instead of a slash as seperator:

`OG 573.2 - 4.6 / 27` --> `OG 573.2 - 4.6 - 27`