In the HTML tables i only compute the aggregated amount by tax class
(e.g., IVA, IRPF), but here we need the actual tax (e.g., IVA 4 %)
because this spreadsheet is intended for accountants.
I can easily extract the amounts from invoice_tax_amount and
expense_tax_amount, but i also need to add the columns to the
spreadsheet, and always with the same order—does not matter much which,
only the same—, that’s why i had to sort the tax IDs when exporting, as
Go does not guarantee an order for maps.
Closes#92