BibFormat: improve Google Scholar support

• Split 'citation_date' into 'citation_publication_date' and 'citation_online_date'. Parse various date formats from either 269c or 260c fields. (closes #1513)

• Added 'citation_pdf_url' meta tag, via new bfe_meta_file.py element.

• Added 'citation_doi' meta tag.

• Accepts comma-separated values for parameter 'tag' in bfe_meta.

• When parsing dates to display as 'citation_publication_date' meta tag, do consider that month names are possibly translated in the current locale.

• Filter out names that are not real authors in 'citation_author' tag.

• First attempt at displaying 'citation_dissertation_institution' meta tag.

Co-authored-by: Theodoros Theodoropoulos <theod@physics.auth.gr>
Co-authored-by: Samuele Kaplun <samuele.kaplun@cern.ch>