USX validation

USX data from the Paratext Data Access API is validated in order to ensure that Scripture Forge does not corrupt Scripture data that it does not know how to properly round-trip. If the DeltaUsxMapper class is updated to support new USX data, then Scripture Forge's USX schema should be updated to reflect the corresponding change. The schema is based on the USX Relax NG schema and can be found at src/SIL.XForge.Scripture/usx-sf.rnc. Once the schema is updated, it must be converted to an XML Schema file. The schema can be converted using Trang. On Linux, install Trang using

sudo apt install trang

Convert the schema to XML Schema format by running

cd src/SIL.XForge.Scripture
trang usx-sf.rnc usx-sf.xsd