Bug 20310: (follow-up) Rename method to get_marc_host

Bug 20310: (follow-up) Rename method to get_marc_host

This method is a mix of database relationship accessor and marc field
accessor. We have get_marc_notes already and other patches in the queue
are also starting to introduce get_marc_* named methods. This patch
updates the method name to conform with that scheme.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>