TST: decouple FITSDataset and SpectralCubeCoordinateHandler to allow …

…inializing this geometry from the stream frontend

yt/geometry/coordinates/spec_cube_coordinates.py

@@ -1,3 +1,5 @@
+from yt.funcs import setdefaultattr
+
 from .cartesian_coordinates import CartesianCoordinateHandler
 from .coordinate_handler import _get_coord_fields
 
@@ -6,6 +8,15 @@ class SpectralCubeCoordinateHandler(CartesianCoordinateHandler):
     name = "spectral_cube"
 
     def __init__(self, ds, ordering=("x", "y", "z")):
+
+        setdefaultattr(ds, "lon_axis", 0)
+        setdefaultattr(ds, "lat_axis", 1)
+        setdefaultattr(ds, "spec_axis", 2)
+        setdefaultattr(ds, "lon_name", "X")
+        setdefaultattr(ds, "lat_name", "Y")
+        setdefaultattr(ds, "spec_name", "z")
+        setdefaultattr(ds, "spec_unit", "")
+
         ordering = tuple(
             "xyz"[axis] for axis in (ds.lon_axis, ds.lat_axis, ds.spec_axis)
         )