[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
Why separating into two methods ? Do everything in _count_amount; default_get will call it and get result[order_id].