Implement XSD validation

[Klortho]

The validator now does RNG validation, and it will be really easy to implement XSD, but there are other, more pressing issues.

Here are some notes.

oXygen has a nice XSD flattener, described here. It's not open source, of course.

To use it on a Mac, for example:

cd /Applications/oxygen
./flattenSchemaMac.sh \
  -in:~/git/jats4r/validator/lib/niso-jats/publishing/1.1d3/oasis-mml3/xsd/JATS-journalpublishing-oasis-article1-mathml3.xsd \
  -outDir:~/temp

It doesn't flatten to one file: there's a limitation with XSD, that you need different files for different namespaces. This flattens from ~50 files → 6.

We'd need to add this list of resultant files to jats/jats.yaml, something like this:

xsd:
  ...
  flat_files:
    - JATS-journalpublishing-oasis-article1-mathml3.xsd
    - mathml3.xsd
    - module-ali.xsd
    - module-oasis.xsd
    - xlink.xsd
    - xml.xsd