Calculates the difference threshold for a
        given difference local maximum.

        Parameters
        -----------
        ymx_i : float 
           The normalized y value of a local maximum.

def __threshold(self, ymx_i):
        """
        Calculates the difference threshold for a
        given difference local maximum.

        Parameters
        -----------
        ymx_i : float 
           The normalized y value of a local maximum.
        """
        return ymx_i - (self.S * np.diff(self.xsn).mean())