Given a Dataset object and selection to be applied returns
        boolean to indicate whether a scalar value has been indexed.

def indexed(cls, dataset, selection):
        """
        Given a Dataset object and selection to be applied returns
        boolean to indicate whether a scalar value has been indexed.
        """
        selected = list(selection.keys())
        all_scalar = all((not isinstance(sel, (tuple, slice, set, list))
                          and not callable(sel)) for sel in selection.values())
        all_kdims = all(d in selected for d in dataset.kdims)
        return all_scalar and all_kdims