BibFormat: several fixes

• The recent changes of get_cited_by that now returns a set broke the intbitset "|" operator, this patch fixes the use of get_cited_by.

• The check for missing formats would raise an exception when the database contains 0 records.

• Instead of a single sql query for updated records, we split it in multiple ones with a limit of 5000 records per query.

• Removes useless variables and imports.

• PEP8 fixes.

Co-authored-by: Alessio Deiana <alessio.deiana@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>