Move spex prism loader to specutils

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

```Python
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)

```

Beginne mit den gezeigten _identify_spex-, identify_spex_prism- und spex_prism_loader-Einstiegspunkten und lies anschließend das zugehörige specutils issue 1259 für den Zielkontext. Die Arbeit ist abgeschlossen, wenn der SpeX-Loader und sein Identifikationsverhalten nach specutils verschoben wurden und das bestehende Verhalten für FITS, Einheiten, Unsicherheiten und Metadaten erhalten bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.