fix: octopus PDF model with public note and subtotal

[thomastraversierpro-alt]

FIX Octopus invoice PDF model: public note position, page break and Subtotals support

Three issues fixed in pdf_octopus.modules.php:

• The public note was printed in the middle of the page instead of its standard position, above the invoice lines table.
• On multi-page invoices, the first line of each new page overlapped the column headers of the table.
• The model was not compatible with the Subtotals module: title lines printed meaningless 0 values in the amount columns, and subtotal lines were neither displayed nor computed.
• The VAT-by-rate table in the Octopus PDF template always displayed "VAT 0%" instead of the actual rate/amount, due to a locally rebuilt VAT array (plain floats) being read afterwards as an associative array.
• The "Reste à payer HT" label in the situation recap was misleading, as the displayed value is actually the TTC amount net of retained warranty. Renamed to "Reste à payer" (FR) / "Remainder to pay".

Before

After

[frederic34]

is it a fix for an external module (subtotal) ?

perhaps make separate PR for each problem's fix

[thomastraversierpro-alt]

The PR now only covers the standard subtotals module, the custom one has been
removed from it.