ebyshev series.

    Convert a z series to the equivalent Chebyshev series. The result is
    never an empty array. The dtype of the return is the same as that of
    the input. No checks are run on the arguments as this routine is for
    internal use.

    Parameters
    ----------
    zs : 1-D ndarray
        Odd length symmetric z-series, ordered from  low to high.

    Returns
    -------
    c : 1-D ndarray
        Chebyshev coefficients, ordered from  low to high.

    r