Find how many of each of the tail pieces is necessary.

        Parameters
        ----------
        mag : `~numpy.ndarray`
            Vector magnitudes; must be non-negative (and an actual ndarray).
        rounding : bool, default: True
            Whether to round or to truncate to the nearest half-barb.
        half, full, flag : float, defaults: 5, 10, 50
            Increments for a half-barb, a barb, and a flag.

        Returns
        -------
        n_flags, n_barbs : int array
            For each entry in *mag*, the number of flags and barbs.
        half_flag : bool array
            For each entry in *mag*, whether a half-barb is needed.
        empty_flag : bool array
            For each entry in *mag*, whether nothing is drawn.
        r