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>
Two kwalitee comments:
* please use `docid` instead of `id` in `foo()`
* please use `dummy_bf = ...` instead of `bf = ...`

After fixing this, you can add to the commit message:
`(PR #2136)`
and:
`Tested-by: Tibor Simko <tibor.simko@cern.ch>`
and repush.  Thanks!