[IMP] sale_margin_percentage: optimize migration script T#78545

[luisg123v]

There's a migration script to recompute sales marging on sale order lines, which was raising memory errors with databases that contain many lines.

The following optimizations are applied:

• Don't call compute method directly but mark the field to be computed. This avoids one write per record and makes the ORM to take care of batches.
• Read currency directly from the line, instead of from the pricelist which in turn requires reading the order.
• Don't read currency if quantity is zero as it won't be used anyway

[CLaurelB]

LGTM 👍