fix warnings

lib/iris/exceptions.py

@@ -350,16 +350,16 @@ class IrisSaverFillValueWarning(IrisMaskValueMatchWarning, IrisSaveWarning):
 
 
 @lru_cache(None)
-def warn_once(msg, type, stacklevel, frame):
+def warn_once(msg, type, stacklevel, frame, **kwargs):
  """Raise a warning only if a similar one has not been raised before."""
- warnings.warn(msg, type, stacklevel=stacklevel)
+ warnings.warn(msg, type, stacklevel=stacklevel, **kwargs)
 
 
-def warn_once_at_level(msg, type=None, stacklevel=0):
+def warn_once_at_level(msg, type=None, stacklevel=0, **kwargs):
  """
  Raise a warning only if a similar one hasn't been raised from the same line
  for a given stack level.
  """
  stacklevel += 1
  frame = traceback.format_stack()[-stacklevel]
- warn_once(msg, type, stacklevel + 1, frame)
+ warn_once(msg, type, stacklevel + 1, frame, **kwargs)