Calculates antennas to flag, based on bad gain and bp solutions.

def calc_flag(self, sig=3.0):
        """ Calculates antennas to flag, based on bad gain and bp solutions.
        """
 
        if len(self.gain.shape) == 4:
            gamp = n.abs(self.gain).mean(axis=0)   # mean gain amp for each ant over time
        elif len(self.gain.shape) == 3:
            gamp = n.abs(self.gain)   # gain amp for selected time

#        badgain = n.where(gamp < gamp.mean() - sig*gamp.std())
        badgain = n.where( (gamp < gamp.mean() - sig*gamp.std()) | gamp.mask)
        self.logger.info('Flagging low/bad gains for ant/spw/pol: %s %s %s' % (str(self.antnum[badgain[0]]), str(badgain[1]), str(badgain[2])))

        badants = badgain
        return badants