Convert frequency in Hz to fractional frequency

    Parameters
    ----------
    frequency: np.array
        Data array of frequency in Hz
    mean_frequency: float
        (optional) The nominal mean frequency, in Hz
        if omitted, defaults to mean frequency=np.mean(frequency)

    Returns
    -------
    y:
        Data array of fractional frequency

def frequency2fractional(frequency, mean_frequency=-1):
    """ Convert frequency in Hz to fractional frequency

    Parameters
    ----------
    frequency: np.array
        Data array of frequency in Hz
    mean_frequency: float
        (optional) The nominal mean frequency, in Hz
        if omitted, defaults to mean frequency=np.mean(frequency)

    Returns
    -------
    y:
        Data array of fractional frequency
    """
    if mean_frequency == -1:
        mu = np.mean(frequency)
    else:
        mu = mean_frequency
    y = [(x-mu)/mu for x in frequency]
    return y