[16.0][IMP]commission, account_commission: settlements grouped by payment date

[matteonext]

This pr allows to generate commissions based on the payment date. For example, if you have three invoices from three different months but with a payment date in the same month, the current behavior generates three separate period, one for each month. With this change, however, all commissions are assigned to a single period. It covers a similar scope but is managed in a different way #581

[OCA-git-bot]

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

[matteonext]

Hi @pedrobaeza, @etobella sorry for bother! I just opened this pr, I maybe need to add some tests and split in two commits, one for each module. What do you think about this changes, would you consider including it? Thanks a lot!