[15.0][IMP] account_operating_unit: allow to pay multiple invoices with different operating units

[AaronHForgeFlow]

That was possible in old versions <v12 (https://github.com/OCA/operating-unit/blob/12.0/account_operating_unit/models/account_payment.py#L71) but it was removed for unknown reason when migrating to v13.

In general, reconciling a bank journal entry with an invoice will require Inter OU balancing entries, as long one OU is paying the balance for the other.

If needed I can add a company parameter for restricting paying invoices for different OU if you want to keep compatibility with the actual code.

@Saran440 @alan196 @ForgeFlow