Envio de factura simplificada resumen TPV a SII

This module adds a way to report a group of simplified invoices (for example, daily POS tickets) to the Spanish SII tax system as a single summary invoice, instead of sending each ticket separately.

On customer invoices and credit notes you get a new checkbox "Is SII simplified invoice Summary?" and two text fields, "First Invoice" and "Last Invoice". These appear on the SII tab of the invoice form and are only shown for customer invoices and credit notes. They can only be edited while the invoice is still in draft; once it is posted they become read-only.

When the invoice is sent to SII, the module changes the invoice type reported: a summary is sent as F4 (summary invoice). If the first and last numbers are the same, it is reported as F2 for a normal customer invoice or R5 for a credit note. When a range is given, the first and last simplified invoice numbers are included in the SII record. The counterparty data is removed from the SII record for these summaries, and the corrective-invoice type is removed when the summary is reported as F4.

Business rules: you cannot create a supplier summary invoice, and the first/last invoice numbers cannot be changed once the invoice has already been sent to SII. A missing VAT on the partner is tolerated for summary invoices.