Extend list of hacks for reading non-standard complied 1d WCS fits files

In the current code, there is a hack to allow "LINEAR" as a spectral
axis. That's clearly not complient with the FITS WCS standard as defined in
https://www.aanda.org/articles/aa/full/2006/05/aa3818-05/aa3818-05.html
and might just be motivated by the GMOS spectrum currently used as
example for this loader.
This code change makes ths list of keywords that will be treated as
spectral extensible and adds "Wavelength" (which is much closer to
"WAVE" as required in the standard than "LINEAR").

specutils/wcs/adapters/fitswcs_adapter.py

@@ -1,8 +1,10 @@
+import warnings
 import astropy.units as u
 from astropy.wcs import (WCS, WCSSUB_CELESTIAL, WCSSUB_CUBEFACE,
                          WCSSUB_LATITUDE, WCSSUB_LONGITUDE, WCSSUB_SPECTRAL,
                          WCSSUB_STOKES, InvalidSubimageSpecificationError)
 
+from astropy.utils.exceptions import AstropyUserWarning
 # Use this once in specutils
 from ...utils.wcs_utils import (convert_spectral_axis,
                                 determine_ctype_from_vconv)
@@ -18,6 +20,8 @@ class FITSWCSAdapter(WCSAdapter):
     wrapped_class = WCS
     axes = None
 
+    substitute_spec_axis_names = ['LINEAR', 'Wavelength']
+
     def __init__(self, wcs):
         super(FITSWCSAdapter, self).__init__(wcs)
         self._spec_axis = None
@@ -71,17 +75,18 @@ def spec_axis(self):
         """
         self._spec_axis = self.wcs.wcs.spec
 
-        if self._spec_axis < 0:
-            try:
-                idx = list(self.wcs.wcs.ctype).index('LINEAR')
-            except ValueError:
+        if (self._spec_axis < 0) and (self._wcs.wcs.spec) < 0:
+            ctypelist = list(self.wcs.wcs.ctype)
+            for n in self.substitute_spec_axis_names:
+               if n in ctypelist:
+                   self._spec_axis = ctypelist.index(n)
+                   warnings.warn("WCS has no standard complient spectral axis, 'ctypes' might not be correct. I'm guessing that axis {} labelled '{}' might be spectral and proceed with that.".format(self._spec_axis, n), AstropyUserWarning)
+                   break
+            else:
                 raise InvalidSubimageSpecificationError(
                     "Cannot find a spectral axis in the provided WCS."
                     "Are your 'ctype's correct?")
 
-            if self._wcs.wcs.spec < 0:
-                self._spec_axis = idx
-
         return self._spec_axis
 
     @property