elsevier_package: add subjects as author keywords

* Adds now subjects from Elsevier as author keywords instead of
  erroneously adding it as standard subject in 65017.

Reported-by: fschwenn <florian.schwennsen@desy.de>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Merge pull request #88 from jalavik/elsevier-topic-keyword-fix

elsevier_package: add subjects as author keywords