pos_package: adds PoS conversion package

* Adds a new package to convert PoS harvested files with
  oaidc format to MARC format.

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
elsevier_package: removed shadowing of TypeError

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>