aps_package: XML format detection

* Adds support for simple XML detection in the APS converter. Raises
  new ApsPackageXMLError should the given XML be of a non-JATS format.

* For performance, ApsPackage allows client to pass pre-defined
  journal mappings.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>