Make handling of extra and global coords by NDCube.to_nddata more general.

And docstring fix.

Author: DanRyanIrish

ndcube/ndcube.py

@@ -1500,7 +1500,7 @@ def to_nddata(self,
             Metadata object of new instance. Default is to use data of this instance.
         psf: Any, optional
             PSF object of new instance. Default is to use data of this instance.
-        extra_coords: `ndcubeextra_coords.ExtraCoordsABC`, optional
+        extra_coords: `ndcube.extra_coords.ExtraCoordsABC`, optional
             Extra coords object of new instance. Default is to use data of this instance.
         global_coords: `ndcube.global_coords.GlobalCoordsABC`, optional
             WCS object of new instance. Default is to use data of this instance.
@@ -1545,9 +1545,11 @@ def to_nddata(self,
         all_kwargs = {key.strip("_"): value for key, value in self.__dict__.items()}
         all_kwargs.update(user_kwargs)
         extra_coords, global_coords = None, None
-        if nddata_type is NDCube:
-            extra_coords = all_kwargs.pop("extra_coords")
-            global_coords = all_kwargs.pop("global_coords")
+        if issubclass(nddata_type, NDCubeBase):
+            if isinstance(all_kwargs["extra_coords"], ExtraCoordsABC):
+                extra_coords = all_kwargs.pop("extra_coords")
+            if isinstance(all_kwargs["global_coords"], GlobalCoordsABC):
+                global_coords = all_kwargs.pop("global_coords")
         # Inspect call signature of new_nddata class and
         # remove unsupported items from new_kwargs.
         nddata_sig = inspect.signature(nddata_type).parameters.keys()