Obtain a unique identifier of the image.

        Parameters
        ----------
        img : astropy.io.fits.HDUList

        Returns
        -------
        str:
             Identification of the image

def get_imgid(self, img):
        """Obtain a unique identifier of the image.

        Parameters
        ----------
        img : astropy.io.fits.HDUList

        Returns
        -------
        str:
             Identification of the image

        """
        imgid = img.filename()

        # More heuristics here...
        # get FILENAME keyword, CHECKSUM, for example...
        hdr = self.get_header(img)
        if 'checksum' in hdr:
            return hdr['checksum']

        if 'filename' in hdr:
            return hdr['filename']

        if not imgid:
            imgid = repr(img)

        return imgid