[IMP] sale_invoice_policy: Several improvements

[rousseldenis]

• [x] Don't use the required parameter to compute the quantity to invoice
• [x] Simplify depends()
• [x] Don't use the required parameter to compute the untaxed amount to invoice
• [x] Don't rewrite the Odoo core compute method
• [x] Improve README

Refactor:

I think the default policy that was applied on sale orders from the default product configuration (defined in sale module) was the wrong approach as it is not contextually correct (products are not sale orders).

That said, I introduced an invoice policy on sale order level more explicitly than before (void) which is 'Products Invoicing Policy'. That let the user free to choose between standard behavior or sale order level configuration.

The question is:

• As the required attribute was used before means that the sale order invoicing policy should not use the product configurations. Should we reintroduce that behavior (to constrain sale orders to NOT use product configuration at all ?) ? @ioans73 @fuentes73 @sergio-teruel @ACheung-FactorLibre @Cedric-Pigeon @acsonefho

[rousseldenis]

Don't hesitate to test it on runboat