switch to ZUGFeRD 2 schematron files

[jstaerk]

Since ZF2 has now been officially released we can use it's schematron files.

How you could help:

• download zugferd 2 infopaket
• check if you can find the referenced files (the version from 2019-03-13 e.g. referred to zugferd2p0_en16931_codedb.xml in Schema/EN16931/zugferd2p0_en16931.sch and to zugferd2p0_extended_codedb.xml in Schema/EXTENDED/zugferd2p0_extended.sch): I did not find them, and I upstreamed the request to the same person who had helped me in a similar issue during the public preview
• merge those files into one schematron each (because we deploy to one jar file and ph-shematron can not load dependencies from resources?). This can be done if you add the file in a namespace, e.g. codes:codedb, add the namespace declaration in schematron root element (xmlns:codes="http://www.mustangproject.org/") and references (e.g. ) and
• update the reference everywhere the old document(filename) reference used to be and
• send me a pull request