Resolve "best_track fails for tracks slices"

Closes #63 (closed)

km3io/rootio.py

@@ -174,7 +174,7 @@ class Branch:
     def __len__(self):
         if not self._index_chain:
             return len(self._branch)
-        elif isinstance(self._index_chain[-1], (int, np.int32, np.int64)):
+        elif isinstance(self._index_chain[0], (int, np.int32, np.int64)):
             if len(self._index_chain) == 1:
                 try:
                     return len(self[:])