elsevier_package: Reworks adding of affiliations

* Adding of affiliations now works in 3 stages
    * Referenced affiliations are added
    * Affiliations in the same author_group are added if there
      were no referenced affiliations
    * Affiliations of higher hierarchies are added

* Check "Tag by Tag The Elsevier DTD 5 Family of XML DTDs" for
  more information.

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>
Merge pull request #83 from mvesper/missing_affiliations

elsevier_package: Reworks adding of affiliations