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>
Merge pull request #18 from GiorgosPa/pos_package

pos_package: various fixes