Implement group_expand for workcenter_id so that all workcenters are visible in the planning by workcenter gantt. Implement some constraint on write because anything can be moved in the gantt view. When changing the workcenter of a workorder, make sure the leave is now set on the new workcenter. closes odoo/odoo#37386 Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>