[Feature] Support for Item Price Allowances (BT-147, BT-148)

[samuliedvin]

Currently, the library does not support the UBL elements for Item price allowances at the line level. These correspond to the following EN 16931 business terms:

• BT-147: Item price allowance amount
• BT-148: Item price base amount

This seems to be relatively straightforward addition by adding a new class member variable to InvoiceLine with type of AllowanceOrCharge, and then adding it to the XML at the end of UblWriter::addLineNode.

My current workaround:

• Use my own extension of the InvoiceLine class to include a AllowanceCharge typed member variable.
• Use my own implementation of the UblWriter to add the cac:AllowanceCharge to the resulting XML.

I’m talking about this element here: https://docs.peppol.eu/poacc/self-billing/3.0/syntax/ubl-invoice/cac-InvoiceLine/cac-Price/cac-AllowanceCharge/