BibDocFile: memory leak in bibdocfiles.BibDoc fix

* Fixes a memory leak in BibDocFile module where BibDoc instances were
  referring to BibDocFiles and viceversa, by using weakref.ref().
  (closes #2082)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Also, a more readable commit headline could be:

`BibDocFile: fix for memory leak in BibDoc`

or 

`BibDocFile: bibdocfiles.BibDoc memory leak fix`