Implementing agnostic version of ParameterGrp for reading XML, replacing XMLReader from src/Base/reader.cpp with new class DocumentReader

src/Base/Persistence.cpp

@@ -61,6 +61,12 @@ void Persistence::Save (Writer &/*writer*/) const
     assert(0);
 }
 
+void Persistence::Restore(DocumentReader &/*reader*/)
+{
+    // you have to implement this method in all descending classes!
+    assert(0);
+}
+
 void Persistence::Restore(XMLReader &/*reader*/)
 {
     // you have to implement this method in all descending classes!