Support for tax lines

[bosd]

Invoices / receipts often have a table near the bottom with a summary of the appied VAT taxes.
To correctly process the invoice in accounting programs we need separatly parse the amount per tax type.

Example receipt:

Example invoice:

Sometimes it is easy to match the tax rate to an invoice line.
In the sample receipt above Tax rate 1 (BTW type in dutch) can be matched to the code 1 of the invoiceline.
code 2 represents 21.00% which can be matched to the last invoice line with the product LPG.
With this usage we are able to reconstruct the over simplified invoicelines to include the VAT information.
Task: Add logic to apply the tax rate on an invoiceline.

In the case when it is not possible to reconstruct the invoiceline. (Example Valk Invoice)
The whole tax summary can be returned in the tax_lines field. So the accounting software can generate a global adjustment lines. So at least the correct taxes can be applied.

Tasks:

• List tax_lines in documentation as standard vields Update TUTORIAL: Add "tax_lines" structure to standard fields #536
• Add logic to apply the tax rate on an invoiceline.