Fix xarray accessor split method repeating same data

Author: kysolvik

dabench/data/_xarray_accessor.py

@@ -48,6 +48,7 @@ def split_train_val_test(self,
         for sl in split_lengths:
             end_i = start_i + sl
             out_ds.append(self._obj.isel(time=slice(start_i, end_i)))
+            start_i = end_i
         return tuple(out_ds)