Fix indexing when numpy integers are used

km3io/rootio.py

@@ -171,7 +171,7 @@ class Branch:
     def __len__(self):
         if not self._index_chain:
             return len(self._branch)
-        elif isinstance(self._index_chain[-1], int):
+        elif isinstance(self._index_chain[-1], (int, np.int32, np.int64)):
             return 1
         else:
             return len(