'One dimensional Box model derivative with respect to parameters'

@classmethod
def fit_deriv(cls, x, amplitude, x_0, width):

    d_amplitude = cls.evaluate(x, 1, x_0, width)
    d_x_0 = np.zeros_like(x)
    d_width = np.zeros_like(x)
    return [d_amplitude, d_x_0, d_width]
