Revert 'remove unnecessary template code from document group'

Reverts commit 06c683dcf3, because used
by the Arch module

src/App/DocumentObjectGroup.cpp

@@ -179,4 +179,16 @@ PyObject *DocumentObjectGroup::getPyObject()
     return Py::new_reference_to(PythonObject); 
 }
 
+// Python feature ---------------------------------------------------------
 
+namespace App {
+/// @cond DOXERR
+PROPERTY_SOURCE_TEMPLATE(App::DocumentObjectGroupPython, App::DocumentObjectGroup)
+template<> const char* App::DocumentObjectGroupPython::getViewProviderName(void) const {
+    return "Gui::ViewProviderDocumentObjectGroupPython";
+}
+/// @endcond
+
+// explicit template instantiation
+template class AppExport FeaturePythonT<App::DocumentObjectGroup>;
+}