[IMP]: added new fields (total, invoiced and remaining amount) while cre...

...ating partial invoice based on sale order

 - Task: https://www.odoo.com/web#id=8891&view_type=form&model=project.task&action=333&active_id=131
 - Pad: https://pad.odoo.com/p/r.G2c8RRFSJWAc2Fko
Multi takes a string, used to group the various fields to compute. Use the method name as multi key. By the way, could you rename _count_amoung by _get_amounts ?