Obtain a summary of information about the image.

def gather_info_hdu(self, hdulist):
        """Obtain a summary of information about the image."""
        values = {}
        values['n_ext'] = len(hdulist)
        extnames = [hdu.header.get('extname', '') for hdu in hdulist[1:]]
        values['name_ext'] = ['PRIMARY'] + extnames

        fits_extractor = self.extractor_map['fits']
        for key in self.meta_dinfo_headers:
            values[key] = fits_extractor.extract(key, hdulist)

        return values