Could not be unmarshalled

Hello,

I tried the jsonix PO example, it works fine with marshalling and unmarshalling.

When I try to implement it in my use case I am getting the following error
`Error: Element [{http://www.w3.org/2001/XMLSchema}xsd:schema] could not be unmarshalled as is not known in this context and the property does not allow DOM content.`

Steps:

1. created INVOICE.js in mappings folder using below command
`java -jar \lib\jsonix-schema-compiler-full-2.3.9.jar -d mappings -p INVOICE https://docs.oasis-open.org/ubl/os-UBL-2.1/xsd/maindoc/UBL-Invoice-2.1.xsd`

2. stored the same xsd (https://docs.oasis-open.org/ubl/os-UBL-2.1/xsd/maindoc/UBL-Invoice-2.1.xsd) as an example in invoice.xml
```
const Jsonix = require('jsonix').Jsonix,
INVOICE = require('../mappings/INVOICE'),
context = new Jsonix.Context([INVOICE]),
unmarshaller = context.createUnmarshaller();

unmarshaller.unmarshalFile('../mappings/invoice.xml',
function (unmarshalled) {
console.log(unmarshalled.value.cac:InvoicePeriod.cbc:StartDate);
});
```

Any help will be highly appreciated. Thanks

PS: code [jsonixInvoice](https://github.com/dishantindept/jsonixInvoice) for reference