Private function.
        Updates the covariance scaling factor ``covar_scale_factor``
        according to its limits

def _update_scale_factor(self, accept_rate):
        '''Private function.
        Updates the covariance scaling factor ``covar_scale_factor``
        according to its limits

        '''
        if accept_rate > self.force_acceptance_max and self.covar_scale_factor < self.covar_scale_factor_max:
            self.covar_scale_factor *= self.covar_scale_multiplier
        elif accept_rate < self.force_acceptance_min and self.covar_scale_factor > self.covar_scale_factor_min:
            self.covar_scale_factor /= self.covar_scale_multiplier