fix dict representation in aggregation

Author: h-mayorquin

src/spikeinterface/core/baserecordingsnippets.py

@@ -560,7 +560,7 @@ def split_by(self, property="group", outputs="dict"):
             recordings = []
         elif outputs == "dict":
             recordings = {}
-        for value in np.unique(values):
+        for value in np.unique(values).tolist():
             (inds,) = np.nonzero(values == value)
             new_channel_ids = self.get_channel_ids()[inds]
             subrec = self.select_channels(new_channel_ids)