elsevier_package: Gets affiliation from sa:affiliation

* Changes the get_authors method to get the affiliation text from the
  sa:affiliation tags (via _affiliation_from_sa_field). According to
  "Tag by Tag The Elsevier DTD 5 Family of XML DTDs" this field is
  required to be present.

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>