Bug 30708: Allow attributes to be multivalued

Bug 30708: Allow attributes to be multivalued

This is not working as it, but we are going to fix the problem when
working on the "Set default values for items added in batch to a train"

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: BULAC - http://www.bulac.fr/
Signed-off-by: Heather Hernandez <heather_hernandez@nps.gov>
Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>