Don't use the 'required' parameter to compute the quantities to invoice or not.
In fact, this should only depends on the invoice policy itself.

As ORM appends new depends() parameters to existing ones, simplify the writing

…he amount

As for the 'qty_to_invoice' field, the 'untaxed_amount_to_invoice' field should not
depends on the required option to compute its value

As we don't want to rewrite Odoo core method inside this module, we
should not rewrite the method to compute the untaxed amount to invoice.

Instead, save the product invoice policy, change it to sale order one without
recomputing context, call the super() method and then, restore its former value.

…eparate dict

…olicy

With default behavior in Odoo, the invoicing policy depends on products configuration.
So, include this as possible value for sale order invoicing policy. Possible values are so:

  - Products Invoice Policy: each product applies its invoicing policy (by sale line)
  - Ordered quantities: the whole sale order will apply the invoicing policy on ordered quantities
  - Delivered Quantities: the whole sale order will apply the invoicing policy on delivered quantities

That said, the 'required' configuration is no more useful as the invoicing policy is now required
on sale order level.

…ecompute()

To avoid marking 'invoice_policy' product field as dirty for products temporary value,
use the working context manager 'protecting()' instead of deprecated 'norecompute() one.

Put the whole process in its own contextmanager.