BibDocFile: allow doctype renaming

• Implements the doctype renaming for a BibDoc.

• Fixes bibdocfile CLI action --set-doctype. (closes #980)

Co-authored-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>