Draft: OWEwoksBaseWidget: attempt to fix trigger_downstream compatibility with non-ewoks widget.

This PR is related to an issue found with darfix refactoring XRD/darfix!314 (merged)

I get some trouble with the following workflow and with the previous darfix branch:

Even if the new data copy is very simple the 'trigger_downstream' fails some how. And the z-sum widget end up with having 'None' instead of a OWSend widget.

From what I can see this is because the object send through the dataset channel of the second link is an ewoks 'Variable' instead of being a OWSendDataset. Nevertheless I am not sure this is the best fix to be done.