Add abstract where Open Access/Licence

[bootsa]

Retrieve Abstracts from CrossRef (or other source) and add to entry.

Need to:

• [x] Pull DOIs from Wikidata and pass to CrossRef
• [x] Check Open Access Status - from Wikidata or CrossRef

Open Questions:

• What about papers that don't have a DOI? What other identifiers are there?
• Are there other sensible sources of (open) Abstracts than CrossRef?

[bootsa]

• 17 adds abstracts but currently doesn't check for licencing info

  • depends on #19