Added warning when using nambu spin + changelog

Signed-off-by: Nick Papior <nickpapior@gmail.com>

CHANGELOG.md

@@ -15,6 +15,8 @@ we hit release version 1.0.0.
       import sisl
       sisl.geom.graphene
 
+- added Nambu spin configuration, this is still experimental
+
 ### Fixed
 - `projection` arguments of several functions has been streamlined
 

src/sisl/physics/spin.py

@@ -8,6 +8,7 @@
 import numpy as np
 
 from sisl._internal import set_module
+from sisl.messages import warn
 
 __all__ = ["Spin"]
 
@@ -100,6 +101,11 @@ def __init__(self, kind: Union[str, int] = "unpolarized"):
                 f"{self.__class__.__name__} initialization went wrong because of wrong "
                 "kind specification. Could not determine the kind of spin!"
             )
+        if kind == Spin.NAMBU:
+            warn(
+                "Using untested Nambu spin-configuration, please be aware "
+                "that this is largely untested code!"
+            )
 
         # Now assert the checks
         self._kind = kind