Simplify abstract syntax further. Added OML exporter and test.

tests/test_xopp2py.py

@@ -0,0 +1,17 @@
+from xopp2py import parser, abstract_syntax
+from xopp2py_oml import oml_writer
+import os
+
+DATADIR = os.path.join(os.path.dirname(__file__), "data")
+
+class DummyOutput:
+    def write(self, text: str):
+        pass
+
+def parse(filename):
+    asyntax = parser.parseFile(os.path.join(DATADIR, filename))
+    oml_writer.writeOML(asyntax, filename, "my_xopp", DummyOutput())
+
+# Just see if these files parse without throwing an exception :)
+parse("SmallXournalFile.xopp")
+parse("TwoHiddenLayers.xopp")