Aged Partner Balance: group by partner, add vat

CHANGELOG:

### CHANGED
- [#592](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/592) Aged Partner Balance: group by partner, add vat

Related cards:

• https://trello.com/c/jkxHgOwb/874-report-saldos-ven%C3%A7uts-dempresa-afegir-informaci%C3%B3
• https://trello.com/c/UnKXPF3y/897-report-saldos-ven%C3%A7uts-dempresa-ampliaci%C3%B3n-de-scope

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