bug fix: keep list of composite keys within next_level dictionary

Author: bsteenwi

graphflex/neighbourhood/extractor.py

@@ -27,7 +27,9 @@ def get_neighborhoods(self, noi, depth):
                                     composite_key = f"{key}¥{nkey}"
                                 else:
                                     composite_key = f"{nkey}"
-                                next_level[composite_key] = value
+                                if composite_key not in next_level:
+                                    next_level[composite_key] = []
+                                next_level[composite_key].extend(value)
 
                 neighbours[d] = next_level
         return neighbours