Expose method to Python to get property name of main geometry

fix inheritance of MeshFeaturePy
fix inheritance of PartFeaturePy
remove useless test() method
implement getPyObject in GeoFeature in case a sub-class doesn't implement it

src/Mod/Part/App/PartFeaturePyImp.cpp

@@ -46,9 +46,3 @@ int PartFeaturePy::setCustomAttributes(const char* , PyObject *)
 {
     return 0; 
 }
-
-PyObject* PartFeaturePy::test(PyObject * /*args*/)
-{
-    PyErr_SetString(PyExc_NotImplementedError, "Not yet implemented");
-    return 0;
-}