dev #13870 Allow biblio records to have multiple genres

Do not stop genre search at first match, but continue until we have no
rules left.
Applies to section and emplacement too.

This patch also fixes the regular expression for extracting subfield
values from iso2709 data by using "positive lookahead" feature.
(see http://www.regular-expressions.info/lookaround.html)
The regexp used to fail when a field contains two consecutive subfields
with the same code. Example: 902 $a value $a another value