Save matplotlib plot of the spectre to a file.

        Args:
            filename: Filename to write to.
            img_format: Image format to use. Defaults to EPS.
            sigma: Full width at half maximum in eV for normal functions.
            step: bin interval in eV

def save_spectre_plot(self, filename="spectre.pdf", img_format="pdf",
                          sigma=0.05, step=0.01):
        """
        Save matplotlib plot of the spectre to a file.

        Args:
            filename: Filename to write to.
            img_format: Image format to use. Defaults to EPS.
            sigma: Full width at half maximum in eV for normal functions.
            step: bin interval in eV
        """
        d, plt = self.get_spectre_plot(sigma, step)
        plt.savefig(filename, format=img_format)