elsevier_package: removes arXiv prefix

* Removes 'arXiv' prefix from old style
  arxiv identifiers.

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Merge pull request #15 from GiorgosPa/pos_package

pos_package: adds PoS conversion package.
Merge pull request #17 from GiorgosPa/elsevier

elsevier_package: removes arXiv prefix
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>

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