Weibull model
    http://www.pisces-conservation.com/growthhelp/index.html?morgan_mercer_floden.htm

    Parameters
    ----------
    x: int
    alpha: float
    beta: float
    kappa: float
    delta: float

    Returns
    -------
    float
        alpha - (alpha - beta) * np.exp(-(kappa * x)**delta)

def weibull(x, alpha, beta, kappa, delta):
    """Weibull model
    http://www.pisces-conservation.com/growthhelp/index.html?morgan_mercer_floden.htm

    Parameters
    ----------
    x: int
    alpha: float
    beta: float
    kappa: float
    delta: float

    Returns
    -------
    float
        alpha - (alpha - beta) * np.exp(-(kappa * x)**delta)
    """
    return alpha - (alpha - beta) * np.exp(-(kappa * x)**delta)