Fixing function ambiguity that is causing model_test ot fail.

As far as I can tell, given the new AddElement variants with an extra optional
bool arg, the compiler was deciding that casting the (also optional, through
multiple function declarations) group_name string could be cast to a bool
to make the call work... resolved by changing the version of the call without
the groupname supplied to have its own special name. (Hopefully this doesn't
affect anyone outside of model_test?