function_name ==
        'Normal': smearing is done by normal distribution.
        'Cauchy': smearing is done by Cauchy distribution.

def set_smearing_function(self, function_name):
        """
        function_name ==
        'Normal': smearing is done by normal distribution.
        'Cauchy': smearing is done by Cauchy distribution.
        """
        if function_name == 'Cauchy':
            self._smearing_function = CauchyDistribution(self._sigma)
        else:
            self._smearing_function = NormalDistribution(self._sigma)