Save the eit data into a eit/sip directory structure

        Parameters
        ----------
        directory: string|path
            output directory

def save_to_eitdir(self, directory):
        """Save the eit data into a eit/sip directory structure

        Parameters
        ----------
        directory: string|path
            output directory
        """
        if os.path.isdir(directory):
            raise Exception('output directory already exists')

        os.makedirs(directory)
        np.savetxt(directory + os.sep + 'frequencies.dat', self.frequencies)

        invmod_dir = directory + os.sep + 'invmod'
        os.makedirs(invmod_dir)
        for nr, key in enumerate(sorted(self.tds.keys())):
            outdir = invmod_dir + os.sep + '{0:02}_{1:.6f}'.format(nr, key)
            self.tds[key].save_to_tomodir(outdir)