Update Relax-NG scheme to the new syntax and adjust all .inx files to follow new and recommended syntax.

This is basically ready and I would've pushed it already.

However there's one test failure in TestHersheyTables.test_all_comparisons that I don't understand and can't say if it was caused by one of my changes (in which case it could only be a parameter that is now set differently as I didn't touch any Python code).

/cc @oskay