Checks Stimulus for any warning conditions

        :returns: str -- warning message, if any, 0 otherwise

def warning(self):
        """Checks Stimulus for any warning conditions

        :returns: str -- warning message, if any, 0 otherwise
        """
        signals, docs, overs = self.expandedStim()
        if np.any(np.array(overs) > 0):
            msg = 'Stimuli in this test are over the maximum allowable \
                voltage output. They will be rescaled with a maximum \
                undesired attenuation of {:.2f}dB.'.format(np.amax(overs))
            return msg
        return 0