added model expression for wavelength solution type none (DC-FLAG == …

…-1) and fixing assertion in test.

specutils/io/default_loaders/wcs_fits.py

@@ -382,15 +382,20 @@ def _none():
     """Required to handle No-wavelength solution
 
     No wavelength solution is considered in the FITS standard (dtype = -1)
-    This method is placed here for completeness even though is not
-    implemented.
 
-    Raises
-    ------
+    This will return the identity function. It does not use
+    `~astropy.modeling.models.Identity` because is not simpler to instantiate.
+    Instead it uses `~astropy.modeling.models.Linear1D`
 
-        NotImplementedError
+    Rretuns
+    -------
+
+        A mathematical model instance of `~astropy.modeling.models.Linear1D`
+        with slope 1 and intercept 0.
     """
-    raise NotImplementedError
+    model = models.Linear1D(slope=1,
+                            intercept=0)
+    return model
 
 
 def _linear_solution(wcs_dict):

specutils/tests/test_loaders.py

@@ -166,8 +166,8 @@ def test_iraf_linear(remote_data_path):
     spectrum_1d = Spectrum1D.read(remote_data_path, format='iraf')
 
     assert isinstance(spectrum_1d, Spectrum1D)
-    assert spectrum_1d.wavelength[0] == 3514.56625402 * u.angstrom
-    assert spectrum_1d.wavelength[100] == 3514.56625402 * u.angstrom + 0.653432383823 * 100 * u.angstrom
+    assert spectrum_1d.wavelength[0] == u.Quantity(3514.56625402, unit='Angstrom')
+    assert spectrum_1d.wavelength[100] == u.Quantity(3514.56625402, unit='Angstrom') + u.Quantity(0.653432383823 * 100, unit='Angstrom')
 
 
 @remote_access([{'id':'3359180', 'filename':'log-linear_fits_solution.fits'}])