Tests if dimension is scalar in each subpath.

def isscalar(cls, dataset, dim):
        """
        Tests if dimension is scalar in each subpath.
        """
        if not dataset.data:
            return True
        ds = cls._inner_dataset_template(dataset)
        isscalar = []
        for d in dataset.data:
            ds.data = d
            isscalar.append(ds.interface.isscalar(ds, dim))
        return all(isscalar)