ner4xml: Include PI/statement from #L2 in the xml file

[BibWiss]

The statement from input.xml

notebooks/ner4xml/data/input.xml

Line 2 in b6d2a63

<?xml-model href="hpl_ao.rng" type="application/xml" schematypens="http://relaxng.org/ns/structure/1.0"?>

will get lost during xml parsing with ElementTree. A solution might include using the lxml library.