Construct a new dataset given the coordinates.

def _make_dataset(cls, coords):
        """Construct a new dataset given the coordinates.
        """
        class Slice(cls._SliceType):
            extra_coords = coords

        Slice.__name__ = '%s.slice(%s)' % (
            cls.__name__,
            ', '.join('%s=%r' % item for item in coords.items()),
        )
        return Slice