fix: [AoU-1287] Add labels for abstract when importing from pubmed
https://issues.unige.ch/browse/AOU-1287
Document with different labels: https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.cgi?db=pubmed&retmode=xml&id=32887634&api_key=e77439eb69bb4ab333a5267c02abb6ce5408
Document without labels in the abstract: https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.cgi?db=pubmed&retmode=xml&id=34352228&api_key=e77439eb69bb4ab333a5267c02abb6ce5408
Edited by Alicia.DeDiosFuente