Tests: Improve METS DC tests
Tests: Improve METS DC tests

Add test for creating repeated elements. Add docstrings.

artefactual/archivematica,sevein/archivematica,artefactual/archivematica,artefactual/archivematica,artefactual/archivematica,sevein/archivematica,sevein/archivematica,sevein/archivematica