scholar: ensure only authors are listed in citation_author

[lnielsen]

Real authors of publications within Invenio are unable to be split from academic advisors/non-authors in the citation_author metatags that Scholar uses for indexing & display in search results:

See for example:

https://ddd.uab.cat/record/55237

includes:

<meta content="López López-Gay, Patricia" name="citation_author" />
<meta content="Parcerisas, Francesc" name="citation_author" />
<meta content="Murcia, Claude" name="citation_author" />

But the page, the author is - Patricia López López-Gay ; directores: Claude Murcia, Francesc Parcerisas

https://ddd.uab.cat/record/99206

includes:

<meta content="Calisto, Bárbara Luísa Machado" name="citation_author" />
<meta content="Fita Rodríguez, Ignasi," name="citation_author" />

On the page, the author is - Bárbara Luísa Machado Calisto ; director: Ignacio Fita Rodríguez

[lnielsen]

Seems like the issue is solved in https://github.com/inveniosoftware/invenio/blob/maint-1.2/modules/bibformat/lib/elements/bfe_meta.py#L94-L105