🧹 Address squeeze deprecated in xarray groupby call

glotaran/optimization/data_provider.py:615: UserWarning: The `squeeze` kwarg to GroupBy is being removed.Pass .groupby(..., squeeze=False) to disable squeezing, which is the new default, and to silence this warning.
    ["".join(sub_arr.values) for _, sub_arr in aligned_groups.groupby("global")]

glotaran/optimization/data_provider.py

@@ -612,7 +612,10 @@ def align_groups(
         # as an alternative to the more elegant xarray built-in which is limited to 32 datasets
         # aligned_group_labels = aligned_groups.str.join(dim="dataset").data
         aligned_group_labels = np.asarray(
-            ["".join(sub_arr.values) for _, sub_arr in aligned_groups.groupby("global")]
+            tuple(
+                "".join(sub_arr.to_numpy().flatten())
+                for _, sub_arr in aligned_groups.groupby("global", squeeze=False)
+            )
         )
 
         group_definitions: dict[str, list[str]] = {}