users require `cei:issued` to describe a copy (`cei:witness`)

Created by: GVogeler

It makes sense to date the copy of document (use case: "Originale, BSNSP 2 AA III22 [A]; copia autentica di 1194 dicembre, BSNSP 2 AA III23 [B]" in the digital edition of the documents of Santa Maria della Grotta) Add reference to the element in cei:witness