Merge UBL Invoice Schematron with CreditNote

Goal

The overall goal is to achieve a more maintainable Schemtron code base by removing as much repitions as possible.

A first step is to remove the separated validation of UBL CreditNote and Invoice, because the syntactic differences are minor.

Steps

except for the rule which matches the root element of an invoice, do everything with or logic similar to how CEN does it.

Material

see https://github.com/itplr-kosit/xrechnung-schematron/blob/master/validation/schematron/ubl-cn/UBL/XRechnung-UBL-model.sch

to find all syntax Elements which differ from UBL Invoice

An example of how this could be done by the european Rules: https://github.com/ConnectingEurope/eInvoicing-EN16931/blob/master/ubl/schematron/UBL/EN16931-UBL-model.sch