add mathml validation script

app/validate_mathml_against_schema.py

+import os
+
+import lxml.etree as et
+
+
+#mathml_object_path = os.path.abspath("mathml_test.xml")
+mathml_object_path = os.path.abspath("iir_filter.xml")
+mathml_schema_path = os.path.abspath("app/report/mathml3/mathml3.rng")
+
+print(f"Expression: {mathml_object_path}")
+print(f"Schema    : {mathml_schema_path}")
+
+
+try:
+    schema = et.RelaxNG(et.parse(mathml_schema_path))
+    expression = et.parse(mathml_object_path)
+except et.XMLSyntaxError as e:
+    print("Provided schema or expression not valid XML. Please check.")
+    print(f"Error log: {e}")
+else:
+    validation_result = schema.validate(expression)
+
+    print(f"Expression {'' if validation_result else '_not_ '}valid according to RelaxNG schema.")
+    if not validation_result:
+        print("Error log:")
+        print(schema.error_log)