Add support for UBL format

[m3nu]

UBL is another invoice standard, besides Factur-x and Zugferd. It's quite popular in the US and outside of Europe.

The specification (sample XML) can be found here for different versions. I'm not sure how different they are between 2.0 and 2.1. We should support both versions.

One problem is that there is no standardized filename, as with Factur-x and Zugferd. So we need a search algorithm that finds all XML files and looks for UBL version strings. This should happen AFTER searching for Factur-x and Zugferd filenames. An example implementation can be found here. Odoo already supports this format.

Required steps:

• [ ] Any sample files available?
• [ ] Add new XML flavor: entries in flavors.yml and required files.
• [ ] Adjust to allow flexible XML filename.
• [ ] Which XMP template to use? This one?
• [ ] Add docs and tests.

[duskybomb]

This zip (for UB 2.1) contains almost every thing we might need, XMLs, XSDs and XSDRT. One thing that isn't there is sample PDFs.

Edit: zip for ubl2.0

[duskybomb]

What is the use of XMP template?

[m3nu]

It defines how the XML is referenced in the PDF. UBL won't have one, so one of the other XMPs needs to be reused with a changed attachment name. At least until an official standard is released.

[nyov]

I would suggest turning that into a different package and not complicating this package with it.