BibFormat: template ctx function prefix changed

* Renames template context function prfix from bfn_ to tfn_

Reported-by: Tibor Simko <tibor.simko@cern.ch>
Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>