The tax balance report searches using a m2m field, which kills performance
in large databases. We need direct SQL queries in order to work around
this issue. See odoo/odoo#30350
* Tests
* PEP8
* Use invoice._convert_to_write(invoice._cache). This way, the onchange will be inheritable and will add here also the added values
* better get_context_values
* unify method compute_balance
* open move lines linked to balance