[IMP] mrp: usability of BoM (lines)

- Task: https://www.odoo.com/web#id=10010&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.FJILGyANBSVkazkw
A 'Group by' on a boolean field is strange, shouldn't we turn this into a regular filter, if we keep it as a boolean?