Return the active scalar's field and name: [field, name]

def active_vectors_info(self):
        """Return the active scalar's field and name: [field, name]"""
        if not hasattr(self, '_active_vectors_info'):
            self._active_vectors_info = [POINT_DATA_FIELD, None] # field and name
        _, name = self._active_vectors_info

        # rare error where scalar name isn't a valid scalar
        if name not in self.point_arrays:
            if name not in self.cell_arrays:
                name = None

        return self._active_vectors_info