update PID-extraction upon metadata "extraction" ( including <dc.relation> by terminology)

upon metadata "extraction", distinguish DOIs from <dc.relation> by EU terminology

• Important: DOI via alternative Identifier:

<dc:relation> info:eu-repo/semantics/altIdentifier/doi/10.1038/ncomms4856 </dc:relation>

prioritization:

1. PIDs found in field "doi
2. PIDs found in field "identifier"
3. PIDs found in field "relation". Validate if any relation contains info:eu-repo/semantics/. If yes, take altIdentifier/doi
4. PIDs found in field "relation" without the block info:eu-repo/semantics/