BibClassify: processing and output decoupling

* Decouples the output rendering from processing so that
  processing is done a second time when asking for specific output
  format.

Tested-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Guillaume Lastecoueres <PX9e@gmx.fr>
18: I101 copyright is missing
46: D400 First line should end with '.', not 'b'
294: D103 Docstring missing