Bug 3216: UNIMARC author facets

Bug 3216: UNIMARC author facets

Add 700$b to UNIMARC author facets.

Other facets subfields could be added now. For example, other subjects
subfields.

Following patches are required to handle better MARC21 subfields and choose
other subfields to deal with UNIMARC format.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Tested under both MARC21 and UNIMARC. Does not cause any regressions with
MARC21, and offers the possibility for better faceting there in the future.
Works as advertised with UNIMARC.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>