Save matplotlib plot to a file.

        Args:
            filename: Filename to write to.
            img_format: Image format to use. Defaults to EPS.
            ylim: Specifies the y-axis limits.

def save_plot(self, filename, img_format="eps", ylim=None,
                  zero_to_efermi=True, smooth=False):
        """
        Save matplotlib plot to a file.

        Args:
            filename: Filename to write to.
            img_format: Image format to use. Defaults to EPS.
            ylim: Specifies the y-axis limits.
        """
        plt = self.get_plot(ylim=ylim, zero_to_efermi=zero_to_efermi,
                            smooth=smooth)
        plt.savefig(filename, format=img_format)
        plt.close()