Move spex prism loader to specutils

Start with the shown _identify_spex, identify_spex_prism, and spex_prism_loader entry points, then read the related specutils issue 1259 for the destination context. Done means the SpeX loader and its identification behavior have been moved to specutils and the existing FITS, unit, uncertainty, and metadata behavior is preserved.

Move Spex loader to specutils. Related issue https://github.com/astropy/specutils/issues/1259

def _identify_spex(filename):
    """
    Check whether the given file is a SpeX data product.
    """
    try:
        with fits.open(filename, memmap=False) as hdulist:
            return "spex" in hdulist[0].header["INSTRUME"].lower() and "irtf" in hdulist[0].header["TELESCOP"].lower()
    except Exception:  # pylint: disable=broad-except,
        return False


def identify_spex_prism(origin, *args, **kwargs):
    """
    Confirm this is a SpeX Prism FITS file.
    See FITS keyword reference at http://irtfweb.ifa.hawaii.edu/~spex/observer/
    Notes: GRAT has values of: ShortXD, Prism, LXD_long, LXD_short, SO_long, SO_short
    """
    is_spex = _identify_spex(args[0])
    if is_spex:
        with fits.open(args[0], memmap=False) as hdulist:
            return (
                isinstance(args[0], str)
                and os.path.splitext(args[0].lower())[1] == ".fits"
                and is_spex
                and ("lowres" in hdulist[0].header["GRAT"].lower() or "prism" in hdulist[0].header["GRAT"].lower())
            )
    else:
        return is_spex


@data_loader("Spex Prism", identifier=identify_spex_prism, extensions=["fits"], dtype=Spectrum)
def spex_prism_loader(filename, **kwargs):
    """Open a SpeX Prism file and convert it to a Spectrum1D object"""

    with fits.open(filename, **kwargs) as hdulist:
        header = hdulist[0].header

        tab = hdulist[0].data

        # Handle missing/incorrect units
        try:
            flux_unit = header["YUNITS"].replace("ergs", "erg ").strip()
            wave_unit = header["XUNITS"].replace("Microns", "um")
        except (KeyError, ValueError):
            # For now, assume some default units
            flux_unit = "erg"
            wave_unit = "um"

        wave, data = tab[0] * Unit(wave_unit), tab[1] * Unit(flux_unit)

        if tab.shape[0] == 3:
            uncertainty = StdDevUncertainty(tab[2])
        else:
            uncertainty = None

        meta = {"header": header}

    return Spectrum(flux=data, spectral_axis=wave, uncertainty=uncertainty, meta=meta)