Temporary check for unsupported NXmx term, undefined_value

AFAIK undefined_value isn't in any existing nexus files.

See nexusformat/definitions#656 for the proposed replacement.

format/nexus.py

@@ -1191,6 +1191,11 @@ def set_frame(pg, transformation):
                     reversed(map(int, nx_detector_module.handle["data_size"][-2:]))
                 )
 
+                # Temporary check for unsupported term. AFAIK undefined_value isn't in any existing nexus files.
+                # See https://github.com/nexusformat/definitions/issues/656 for the proposed replacement.
+                assert "undefined_value" not in nx_detector.handle, \
+                  "undefined_value not an NXmx term. See nexusformat/definitions#656."
+
                 # Get the trusted range of pixel values
                 underload = (
                     float(nx_detector.handle["undefined_value"][()])