Add open access info in field 506 #43

TobiasNxrequested to merge
43-oaStatements into main

Digibib/Introx asked for additional Open Access info.

Following the DINI KIM best practice 3.1: https://wiki.dnb.de/pages/viewpage.action?pageId=330012514

Related to #43 (closed)

We have no labels for 540 so we stick to the URI.

@acka47 Could you have a look? Also @blackwinter could you have a look.

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