Export to IRIS Instrument configuration file

        Parameters
        ----------
        filename : string
            Path to output filename

def to_iris_syscal(self, filename):
        """Export to IRIS Instrument configuration file

        Parameters
        ----------
        filename : string
            Path to output filename
        """
        with open(filename, 'w') as fid:
            # fprintf(fod, '#\t X\t Y\t Z\n');
            fid.write('#\t X\t Y\t Z\n')
            # fprintf(fod, '%d\t %.1f\t %d\t %d\n', D');
            # loop over electrodes and assign increasing x-positions
            # TODO: use proper electrode positions, if available
            for nr in range(0, self.configs.max()):
                fid.write('{}   {}  0   0\n'.format(nr + 1, nr))

            # fprintf(fod, '#\t A\t B\t M\t N\n');
            fid.write('#\t A\t B\t M\t N\n')

            # fprintf(fod, '%d\t %d\t %d\t %d\t %d\n', C');
            for nr, config in enumerate(self.configs):
                fid.write('{}  {}  {}  {}  {}\n'.format(nr + 1, *config))