Schema coverage testing

[jwfraustro]

Currently, the pydantic-xml models in this package are hand-written based on the relevant standard's XSD schema. With them are unit tests that cover the basic round-tripping from model to XML, and that the various attributes are accessible as expected, especially when discriminating between inherited classes.

However, this leaves room for parts of the schema to be missed due to human error. Some type of automated coverage checking of the current models against the standard's schema would provide some insulation from this.