XML schema validation

It is possible to manually perform extra validation of a fattura using the official XML Schema Definition with xmlstarlet:
```
xmlstarlet val  -e -s  doc/Schema_del_file_xml_FatturaPA_versione_1.2.1.xsd  filename.xml
```

This could possibly be integrated into a38/

The `xsd` used is downloaded with the `download-docs` script in the a38 git repository