Selector now also works correctly on lists.

src/mdevaluate/correlation.py

@@ -99,20 +99,18 @@ def shifted_correlation(
             return get_correlation(frames, start_frame, index, shifted_idx)
         else:
             index = selector(frames[start_frame])
-            if index.ndim == 1:
+            if len(index) == 0:
+                return np.zeros(len(idx))
+
+            if isinstance(index[0], int) or isinstance(index[0], bool):
                 return get_correlation(frames, start_frame, index, shifted_idx)
-            elif index.ndim == 2:
+            else:
                 correlations = []
                 for ind in index:
                     correlations.append(
                         get_correlation(frames, start_frame, ind, shifted_idx)
                     )
                 return correlations
-            else:
-                raise ValueError(
-                    f"Index list of selector has {index.ndim} dimensions, "
-                    "but should have 1 or 2"
-                )
 
     if 1 - skip < window:
         window = 1 - skip