Blank call code shown for some multi-volume works

[danmichaelo]

Some series objects do not have their own call code, but rather a "(Plassering: Se de enkelte bind)" in "Utlånsstatus / merknad". See Regional geology and tectonics for an example.

To start with, I think displaying "Utlånsstatus / merknad" is a good (and simple) idea just to give the user a clue on what's going on. Next, displaying a linked list of the objects in the series could be considered.