[16.0][IMP] sale_order_invoicing_grouping_criteria: use odoo's grouping functions

[Tisho99]

Odoo already has a grouping function that can be simply inherited to add more grouping criterias.

The function is declared in the sale module, and its used inside the _create_invoices function to group the values of the new invoices.

The module sale_order_invoicing_grouping_criteria currently uses a new custom way to group invoices.

Benefits of changing the grouping method:

• Reduce the amount of customization and code
• More compatibility with other odoo and oca modules

Cons of changing the grouping method:

• Agrupation can only be done though fields present in the generated invoice. Fields only present in the sale, for example: sale id, can not be used in the agrupation (Thats why a test is commented, pending to solve)

T-6942

[OCA-git-bot]

Hi @pedrobaeza,
some modules you are maintaining are being modified, check this out!