pos_package: various fixes

* Removes field 786.

* Adds field 980__a:HEP.

* Switces content from 520__a to 500__a.

* Adds Docstrings.

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
elsevier_package: removes shadowing of TypeError.

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Merge pull request #18 from GiorgosPa/pos_package

pos_package: various fixes
Merge pull request #19 from GiorgosPa/elsevier

elsevier_package: removes shadowing of TypeError.
elsevier_package: print function and ext_link fix

* Uses print as a function for future Python 3 compatibility.

* Fixes problem with ext_link when it sometimes was None.

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