convert the Schema to schematron and save at the given output filename or with the given extension.

def schematron(self, fn=None, outfn=None, ext='.sch'):
        """convert the Schema to schematron and save at the given output filename or with the given extension."""
        from .xslt import XSLT
        from . import PATH, XML, etree
        fn = fn or self.fn
        if os.path.splitext(fn)[-1].lower()==ext:
            return fn
        elif os.path.splitext(fn)[-1].lower()!='.rng':
            fn = Schema(fn=fn).trang(ext='.rng')
        rng2sch = XSLT(fn=os.path.join(PATH, 'xslts', 'rng2sch.xslt'))
        rng = XML(fn=fn)
        outfn = outfn or os.path.splitext(fn)[0]+ext
        sch = XML(fn=outfn, root=rng2sch.saxon9(rng.root).getroot())
        sch.write()
        return sch.fn