BibDocFile: bibdocfile.BibDoc memory fix

* Fixes a memory leak in BibDocFile module where BibDoc instances were
  referring to BibDocFiles and viceversa, by using weakref.ref().
  (closes #2082) (PR #2136)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Tested-by: Tibor Simko <tibor.simko@cern.ch>