ZUGFeRDInvoiceImporter: Item-Allowances not imported

[danl-devp]

Allowances for Items are not available after Import.

Steps:

1. Export Invoice

                Invoice invoiceOut = new Invoice();

                [...]

		Item itemOut = new Item();
                [...]
		itemOut.addAllowance(new Allowance(BigDecimal.valueOf(123)));

		var zf2p = new ZUGFeRD2PullProvider();
		zf2p.setProfile(Profiles.getByName("XRechnung"));
		zf2p.generateXML(invoiceOut);
		byte[] xml = zf2p.getXML();

2. Import from xml

                ZUGFeRDInvoiceImporter importer = new ZUGFeRDInvoiceImporter();
		importer.fromXML(new String(xml));

		Invoice invoiceIn = importer.extractInvoice();

		for (var itemIn : invoiceIn.getZFItems())
		{
			IZUGFeRDAllowanceCharge[] itemAllowances = itemIn.getItemAllowances();
			assert itemAllowances.length > 0;
		}

• The data for Allowances is stored in Xml-Element GrossPriceProductTradePrice.
• The class responsible for importing an item is org.mustangproject.Item.
• During the import process this Xml-Element is not considered.